Add Danger bot rules on GitLab project for stable branches

As part of Improve patch release process in preparation fo... (&828 - closed), maintainers will be allowed to merge bug fixes into GitLab stable branches. Care should be taken to avoid merging invalid code changes (e.g. features), for that, an automation should be implemented to guide developers into this new merging process.

Merge requests on GitLab are validated by Danger bot, e.g gitlab-org/gitlab!104403 (comment 1176282433)

Example: gitlab-org/gitlab!104403 (comment 1176282433)

We could reuse these bot rules and extend them to consider stable branches:

• Danger bot should fail if the merge request:
  • Is labeled with feature
  • Is not labeled with type::bug
  • Targets an unsupported branch, e.g. merging to 15-5-stable-ee is okay, but merging into 15-1-stable-ee is not.
• When the Danger bot fails it should notify the merge request author about it.
• These rules should be ignored on preparation branches (this is only for the interim period as not to interrupt the current patch release process)
• These rules should only apply to GitLab canonical project (not security)