[API Compat] Adding an ObsoleteAttribute or ExperimentalAttribute is not detected as a breaking change

[bitbonk]

Is your feature request related to a problem? Please describe.

When adding the ObsoleteAttribute or ExperimentalAttribute to an existing public API, these changes are not detected as a source breaking changes by the baseline package validator. Such changes are not source compatible because the former would result in a new compiler warning in consuming code and the latter even a compiler error.

Describe the solution you'd like

Introduce two new validation checks. One for ObsoleteAttribute and one for ExperimentalAttribute that fails when the attribute is added to a public API that existed without that attribute in the basline version without the attribute.

Additional context

In .NET 9, obsoletions where also documented as source-incompatible breaking changes.

[dotnet-policy-service]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage