[release/5.0-rc2] Re-add linker attributes to generic type params of JsonSerializer methods

[github-actions]

Backport of #42186 to release/5.0-rc2

/cc @layomia

Customer Impact

Testing

Risk

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[danmoseley]

Needs template filling out.

[layomia]

cc @vitek-karas @MichalStrehovsky - is porting this change needed for .NET 5? These annotations were mistakenly dropped in #41484 due to attributes on generic type parameters not being supported by /t:GenerateReferenceSource (tracked in dotnet/arcade#5925).

AFAIK, the change has no impact for customers. When customers run the linker in 5.0, it will always be against the implementation assemblies.

[MichalStrehovsky]

I don't think this is critical to backport - this annotation is incomplete anyway because it doesn't deal with the recursion that System.Text.Json will inavoidably do (dotnet/linker#1087 talks about that). We'll need to do something with this in .NET 6.

[vitek-karas]

I agree - not critical to backport.

[layomia]

Thanks. Closing as this isn't critical for 5.0 cc @eerhardt.