[release] Prepare release core-1.12.1-alpha.2

[Kielek]

Note: This PR was opened automatically by the prepare release workflow.

Requested by: @Kielek

Changes

• CHANGELOG files updated for projects being released.

Commands

/UpdateReleaseDates: Use to update release dates in CHANGELOGs before merging [approvers, maintainers]
/CreateReleaseTag: Use after merging to push the release tag and trigger the job to create packages [approvers, maintainers]
/PushPackages: Use after the created packages have been validated to push to NuGet [maintainers]

[Kielek]

/UpdateReleaseNotes

• TESTBreaking Change2: OpenTelemetry.Exporter.OpenTelemetryProtocol now
  defaults to using OTLP/HTTP instead of OTLP/gRPC when targeting .NET Framework
  and .NET Standard. This change may cause telemetry export to fail unless
  appropriate adjustments are made. Explicitly setting OTLP/gRPC may result in a
  NotSupportedException unless further configuration is applied. See
  #6209 for
  full details and mitigation guidance. #6229

[Kielek]

/UpdateReleaseNotes

• Breaking Change3: OpenTelemetry.Exporter.OpenTelemetryProtocol now
  defaults to using OTLP/HTTP instead of OTLP/gRPC when targeting .NET Framework
  and .NET Standard. This change may cause telemetry export to fail unless
  appropriate adjustments are made. Explicitly setting OTLP/gRPC may result in a
  NotSupportedException unless further configuration is applied. See
  #6209 for
  full details and mitigation guidance. #6229