Ensure the stable branch template is used in backports

Context

Per the GitLab Maintenance Policy, bug fixes can be backported to the current version at any time. There is a runbook with the required steps to backport a bug fix into the current version. The runbook requires every backport to use the stable branch template, however, this step can be easily missed because it is not enforced, which requires release managers to point that out (example).

Proposal: Update Danger to ensure backports always use the respective template

Lets remove the manual overhead from release managers and automate this check with Danger. The danger-review job runs on GitLab merge requests and it is used to point out different required checks (example). It can be expanded to verify if the respective stable branch is used and if not fail the job.

The logic is likely to be implemented on the stable_branch.rb file

Exit criteria

• The check is automated with Danger. Danger fails if the stable branch template is not used in backports
• The merge request is backported to older versions, ideally up to 16.0