[release/5.0] Correct baseline checks

- mostly duplicates #25217
- update `BaselineGenerator` to produce baselines useful in 6.0 (too)
- update Baseline.Designer.props using new generator (matching 3.1.7 release)
- always suppress references expressed only in `*.nuspec` files
  - needed even in servicing builds
- restore warning and errors about removed references (new for 5.0)
  - adjust exclusions to handle `@(_ProjectReferenceByAssemblyName)` removal

nit: do not generate empty `<ItemGroup />` elements

Author: dougbu