[Feature Request] Support building with preview versions of the .NET SDK

**Is your feature request related to a problem? Please describe.**

Testing applications out with the .NET 9 Preview 1 SDK, it's not possible to build sites using docfx that have a `net9.0` target framework.

This appears to be due to the code below, which requires that the SDK used is the same version as the runtime version docfx uses.

https://github.com/dotnet/docfx/blob/bebcd35323da35e253125c5b3cfd12e7a4553696/src/Docfx.Dotnet/CompilationHelper.cs#L135-L159

Even if the .NET 8 SDK is installed, in addition to .NET 9's, and `DOTNET_ROLLFORWARD=Major` is set, docfx will still and try to use the .NET 8 SDK to build for .NET 9, which fails.

```console
Using .NET Core SDK 8.0.200
Loading project 
/runner/_work/MyProject/MyProject/src/MyProject/MyProject.csproj
  Determining projects to restore...
Error: /usr/share/dotnet/sdk/8.0.200/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(166,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 9.0.  Either target .NET 8.0 or lower, or use a version of the .NET SDK that supports .NET 9.0. Download the .NET SDK from https://aka.ms/dotnet/download [/runner/_work/MyProject/MyProject/src/MyProject/MyProject.csproj::TargetFramework=net9.0]
```

**Describe the solution you'd like**

docfx allows the use of newer .NET SDKs if installed in the execution environment.

**Describe alternatives you've considered**

Wait until November until .NET 9 is released and ignore my failing documentation builds.

**Additional context**

None.


	private static IEnumerable<MetadataReference> GetDefaultMetadataReferences(string language)
	{
	try
	{
	// Get current .NET runtime version with `{major}.{minor}` format.
	var dotnetMajorMinorVersion = Environment.Version.ToString(2);

	// Resolve .NET SDK packs directory path. (e.g. `C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\8.0.2\ref\net8.0`)
	var dotnetExeDirectory = DotNetCorePathFinder.FindDotNetExeDirectory();
	var refDirectory = Path.Combine(dotnetExeDirectory, "packs/Microsoft.NETCore.App.Ref");
	var version = new DirectoryInfo(refDirectory).GetDirectories().Select(d => d.Name).Where(x => x.StartsWith(dotnetMajorMinorVersion)).Max()!;
	var moniker = new DirectoryInfo(Path.Combine(refDirectory, version, "ref")).GetDirectories().Select(d => d.Name).Where(x => x.EndsWith(dotnetMajorMinorVersion)).Max()!;
	var path = Path.Combine(refDirectory, version, "ref", moniker);

	Logger.LogInfo($"Compiling {language} files using .NET SDK {version} for {moniker}");
	Logger.LogVerbose($"Using SDK reference assemblies in {path}");
	return Directory.EnumerateFiles(path, "*.dll", SearchOption.TopDirectoryOnly)
	.Select(CreateMetadataReference);
	}
	catch (Exception ex)
	{
	Logger.LogVerbose(ex.ToString());
	throw new DocfxException("Cannot find .NET Core SDK to compile the project.");
	}
	}

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[Feature Request] Support building with preview versions of the .NET SDK #9745

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[Feature Request] Support building with preview versions of the .NET SDK #9745

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