Update NonVersionableAttribute attribute trimming

[marek-safar]

No description provided.

[dotnet-issue-labeler]

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.

[MichalStrehovsky]

More of a general question, but since you're touching this file - should we move this to the shared location and start making these conditional on feature switches? For example the entry for System.ObsoleteAttribute at the beginning has comments that this might be sketchy, but it's an okay default for WASM. In .NET 6, Mono's CoreLib won't be WASM only anymore.

Would be nice to have them as feature switches. Blazor/WASM can default those to on by default. Xamarin can also make their own choices. Desktop workloads would probably default them all to "kept", but it's a nice opt-in to have.

[marek-safar]

I'm not against someone porting everything or a subset of this to a shared location but it'd be better done separately. Right now any Mono workload defaults to these settings without feature switches.

[MichalStrehovsky]

Ok, I filed #48217. I put it as Milestone 6.0.0 because we likely don't want to ship a version of Xamarin where Entity Framework doesn't work right after trimming.