Better guide backport MR authors to reviewers and mergers

Context

From gitlab-com/content-sites/handbook!15114 (comment 2670834474)

As we extend the maintenance policy to allow bug fixes to be backported further, we should implement better guide for backport MR authors to respective reviewers and mergers.

Guidelines

For MRs created on the stable branches for versions in the extended maintenance policy:

• The reviewer and merger can be the same
• Merger just needs to be a maintainer of the repository
• Preferably, the reviewer + merger should be the same one as the default branch MR.

Implementation suggestions

• Danger message
• Make sure/enable the Reviewer suggestions are available on canonical merge requests targeting stable branches

Exit Criteria

• Create MR to notify the MR authors of the above guidelines when they create backport MRs to stable branches in the maintenance policy