Bug

Fixes: infrastructure

Description

Working on NuGet.CommandLine.XPlat has been a bit painful the last few months, because all my test projects I create using dotnet new will target .NET 10, but when XPlat is targeting net9.0, then MSBuild locator will load the .NET 9 SDK, and then MSBuild's APIs to load project files fail because the .NET 9 SDK can't target net10.0.

So, this PR:

• Changes the version of the .NET SDK we use to build NuGet
• Change the LangVersion to C# 14, so we can use the latest net10.0 language features. I'm particularly looking forward to using the field keyword
• Retarget XPlat, and a few test projects that use the same property, to net10.0
• Fix some build errors that popped up. I'm not sure I've seen errors pop up when upgrading the LangVersion before, but I guess this is exactly why that setting exists.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A