[ApiCompat] Warn if abstract keyword is removed from member

[carlossanlop]

Fixes #46794

Removing the abstract keyword from a member should be disallowed by ApiCompat, but it currently does not know how to handle that case like it used to in the old version.

The lack of warning in ApiCompat caused the introduction of this bug in maintenance-packages System.Reflection.DispatchProxy: dotnet/maintenance-packages#194 which we fixed with dotnet/maintenance-packages#202

[ViktorHofer]

Does this also account for strict mode in which the API members should be equal?

[carlossanlop]

Does this also account for strict mode in which the API members should be equal?

Good catch, I added a test for strict mode removal of abstract.

[carlossanlop]

@ViktorHofer @ericstj how does this flow into maintenance-packages?

[ericstj]

Either through a new SDK, or we can selectively update the APICompat package. We have to be careful in updating the APICompat package in case it references a newer set of dependencies that are available in the SDK used by m-p.

What we've said in the past is we wanted to keep m-p on the LTS SDK- which would mean we need to backport this change to 8.0.4xx. Not sure we want to do that with this fix, since it will create a new warning for existing customers.

[ViktorHofer]

Yes, adding an OOB PackageReference to it in m-p should work. We have done that in the past.