APICompat: Log assembly reference resolve failures as warnings again

[ViktorHofer]

This is necessary to unblock dotnet/runtime#111328

We should find a better heuristic so that we only log assembly reference load warnings when those assembly references are actually required for the assembly under test.

This affects both APICompat and GenAPI.

[dotnet-policy-service]

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