Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 warnings due to
    nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 now requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 now requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 now requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Update `SamplesApp.UITests.csproj` to explicitly use
    Microsoft.NET.Test.Sdk 17.14.1, to "override" the nearly four
    year old Microsoft.NET.Test.Sdk 17.0.0 reference implicitly
    brought in from Uno.UITest.Helpers/1.1.0-dev.70.

    This hopefully fixes a `MissingMethodException` for
    `IFileSystem.Exists(string)` seen on CI:

        Unhandled exception. System.MissingMethodException: Method not found: 'Boolean Microsoft.Testing.Platform.Helpers.IFileSystem.Exists(System.String)'.
           at Microsoft.Testing.Extensions.VSTestBridge.Configurations.RunSettingsConfigurationProvider.BuildAsync(CommandLineParseResult commandLineParseResult)
           at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
           at Microsoft.Testing.Extensions.VSTestBridge.Configurations.RunSettingsConfigurationProvider.BuildAsync(CommandLineParseResult commandLineParseResult)
           at Microsoft.Testing.Platform.Configurations.ConfigurationManager.BuildAsync(IFileLoggerProvider syncFileLoggerProvider, CommandLineParseResult commandLineParseResult) in /_/src/Platform/Microsoft.Testing.Platform/Configurations/ConfigurationManager.cs:line 37
           at Microsoft.Testing.Platform.Hosts.TestHostBuilder.BuildAsync(ApplicationLoggingState loggingState, TestApplicationOptions testApplicationOptions, IUnhandledExceptionsHandler unhandledExceptionsHandler, DateTimeOffset createBuilderStart) in /_/src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.cs:line 131
           at Microsoft.Testing.Platform.Builder.TestApplicationBuilder.BuildAsync() in /_/src/Platform/Microsoft.Testing.Platform/Builder/TestApplicationBuilder.cs:line 112
           at TestingPlatformEntryPoint.Main(String[] args) in /__w/1/s/src/SamplesApp/SamplesApp.UITests/obj/Release/net9.0/TestPlatformEntryPoint.cs:line 14
           at TestingPlatformEntryPoint.<Main>(String[] args)

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    This can make it annoyingly difficult to `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    This can make it annoyingly difficult to `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.

Context: #19934
Context: #21140
Context: dotnet/sdk#48949

#19934 was the first attempt to build unoplatform/uno
with .NET 10.  It *built* (mostly), but it had *lots* of unit test
failures (over 3700) that were not understood at the time.

#21140 was an attempt to cleanup & simplify #19934.
It partially succeeded, but required additional reconsideration for
a cleaner and more understandable merge.

(Later, PR #21183 helped explain many of the unit test failures that
PR #19934 observed: Xamarin.UITest doesn't work reliably under
.NET 9! See microsoft/appcenter#2646 and microsoft/appcenter#327!)

Some of the contents of #19934 and #21140 have been split out into
separately merged PRs such as #21230 (CS0246 fixes)
and #21199 (remove `[Export]` in some scenarios).

Squash existing "known good" and related changes:

  * doc updates

  * Bumping target framework versions,

  * Use uno.check 1.32.0-dev.45, which knows about .NET 10 Preview 7.

  * `$(TreatWarningsAsErrors)`=true +
    NuGet warnings.  For example:

        error NU1510: Warning As Error:
        PackageReference System.Private.Uri will not be pruned.
        Consider removing this package from your dependencies, as it is likely unnecessary.

  * `$(TreatWarningsAsErrors)`=true + CS8604 or CS0809 warnings due
    to nullability changes, particularly in iOS.

  * `$(TreatWarningsAsErrors)`=true + IL2\* warnings, some of which
    @jonpryor was able to fix in a reasonable timeframe, and some of
    which was thrown into `$(NoWarn)` to deal with later.

  * Remove hardcoded `$(TargetFramework)`=net8.0 from
    `Uno.UI.RemoteControl.TestProcessor.csproj`, so that it builds,
    and remove other uses of `net8.0` from everywhere other than
    `src/SolutionTemplate`.

  * Provide `%(PackageReference.Version)` for packages that were
    producing NU1015 errors, specifically `Uno.Fonts.OpenSans`.

  * Remove `@(PackageReference)` values which produced NU1510
    warnings such as:

        warning NU1510: PackageReference Microsoft.Win32.Registry will not be pruned. Consider removing this package from your dependencies, as it is likely unnecessary.

  * Improve `dotnet-install.yml` use, as some CI environments *don't*
    have a `unoplatform/uno` checkout, and thus *cannot* copy files
    such as `build/ci/net10/global.json`.

  * Improve `local-android-uitest-run.sh` for local macOS use:
    Export and use `$ANDROID_AVD_HOME`, so that we have a "known"
    location for whare Emulator images are created.  (Not sure what's
    wrong with my environment, but they are not reliably created or
    loaded from `$HOME/.android/avd`.)

    Also update so that it doesn't require running the script from the
    `build/test-scripts` directory.  You can now run it from topdir:

        UITEST_IS_LOCAL=true \
          build/test-scripts/local-android-uitest-run.sh

  * Provision OpenJDK-17, as .NET 9 requires it.

  * Use Xcode 16.4, as .NET 10 iOS now requires it.

  * Add new `$(NetPreviousNetCoreApple)` MSBuild property to
    `Directory.Build.props`, which contains target frameworks for
    $(NetPrevios) Apple platforms.

    Update `Uno.UI.Runtime.Skia.AppleUIKit.csproj` to use
    `$(NetPreviousNetCoreApple)`, and update `skia-ios-uitest-build.sh`
    to provide a "complete" target framework version.

    This is so that target frameworks are consistent:
    `net9.0-ios` *floats*; circa a year ago, it aliased
    `net9.0-ios18.0`, and *today* it aliases `net9.0-ios18.5`.
    "Even better", this doesn't "flow" to the C# compiler:
    `net9.0-ios` sets `IOS18_0_OR_GREATER` *but not*
    `IOS18_5_OR_GREATER` *even when* the 18.5 assemblies are used!
    This can make it annoyingly difficult to get `[Obsolete]`s
    consistent across separate project builds.

TODO, for future commits to this PR:

  * Xamarin.AndroidX package version bumps from #21205.
    We felt that these shouldn't be separately merged, as some of
    version bumps are quite significant and should instead only be
    used with .NET 10.

  * A new understanding of `src/SolutionTemplate`: the intent, as
    @jonpryor currently understands it, is that tests within
    `src/SolutionTemplate` are "frozen in time"; `$(TargetFrameworks)`
    should *not* be updated, etc.

    Which means that when we drop support for an older version of .NET,
    we should in turn *remove the old tests*.