Remove netstandard1.x assets from SBRP

[ViktorHofer]

Many projects in the source-build-reference-packages repository currently target netstandard1.x. Targeting netstandard1.x is now discouraged with the latest .NET SDK and we should remove those from SBRP.

This issue is blocked until all repositories that consume SBRP removed their netstandard1.x assets.

[ViktorHofer]

@MichaelSimons would you mind sharing an up-to-date list of the netstandard1.x packages that are still consumed and the components that bring them in?

[ViktorHofer]

These repos still bring netstandard1.x packages in:

• Runtime: Only build the most recent source generator when building from source runtime#104139
• Roslyn: Don't reference netstandard1.x packages roslyn#74199 & dotnet/roslyn@5998125 & Set FlagNetStandard1XDependencies=true roslyn#74549
• NuGet: Remove netstandard1.x dependencies in source-build NuGet/NuGet.Client#5885
• Roslyn-analyzers
• Sdk: Define "ToolingPackageVersion" properties for msbuild tooling dependencies sdk#43008
• Source-build-externals
  • Update msbuild version to eliminate NetStandard1.* references source-build-externals#342
  • Remove NetStandard1.0 tfm support from Humanizer source-build-externals#343
  • Stop buildinging NewtonSoft.Json for NetStandard1.* source-build-externals#344
  • Eliminate netstandard1.* references from application-insights source-build-externals#345
• VSTest: Remove netstandard1.x dependencies microsoft/vstest#5191

[MichaelSimons]

Here is a report of the current netstandard1.x package usage.
ns1.usage.2024.7.3.txt

[ViktorHofer]

Repositories that eliminated their netstandard1.x packages (for at least source-build) should then set this switch so that this doesn't regress over time: https://github.com/dotnet/roslyn/blob/dabd07189684b5cda34b3072326a12b18301a012/eng/Versions.props#L86-L87.