Add Danger message to guide backport MR authors to reviewers and mergers

What does this MR do and why?

Related issue: gitlab-com/gl-infra/delivery#21434 (closed)

This change adds helpful guidance for reviewers working on stable branch merge requests (code changes that fix bugs in stable branches within the maintenance policy).

When someone submits a fix targeting a version that's still being maintained, the automation now displays a message explaining that the same person can both review and merge the change, and that they must be a maintainer. The message also suggests using the same reviewer as the original fix and provides a link to help find available reviewers.

For older versions that are no longer maintained, it continues showing the existing warning message instead of the new guidance.

The tests verify this new behavior works correctly - showing the guidance message for maintained versions and not showing it for unmaintained ones.

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