Remove extra model annotations from snapshot

[AndriySvyryd]

Put all the annotations that should be removed in one place that CSharpSnapshotGenerator, MigrationsCodeGenerator and CSharpDbContextGenerator use.

And add tests for ignored model annotations

[ajcvickers]

Putting this on the backlog to make sure that we have test coverage and that the correct annotations are included in the snapshot.

Note that in triage we were not 100% sure whether MaxIdentifierLength should be in the snapshot or not. There may have been MySQL bugs when it wasn't there.

[roji]

Poaching to do as part of the work for #21210