[release] Prepare release core-1.15.3

[otelbot-dotnet]

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

Requested by: @Kielek

Changes

• CHANGELOG files updated for projects being released.
• Public API files updated for projects being released (only performed for stable releases).

Commands

/UpdateReleaseDates: Use to update release dates in CHANGELOGs before merging [approvers, maintainers]
/UpdateReleaseNotes: Use to update RELEASENOTES.md 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]

[otelbot-dotnet]

I noticed this PR is releasing a stable version of core packages but there isn't any content in RELEASENOTES.md for the version being released.

It is important to update RELEASENOTES.md before creating the release tag because a permalink will become part of the package(s).

Post a comment with "/UpdateReleaseNotes" in the body if you would like me to update release notes.

Note: In the comment everything below "/UpdateReleaseNotes" will be added to RELEASENOTES.md for the version being released. If something is already there it will be replaced.

[Kielek]

/UpdateReleaseDates

[otelbot-dotnet]

I updated the CHANGELOG release dates.