Remove branch-related packages on merge

Release notes

You want to spend your time writing code, not thinking about storage management. If you are creating and publishing packages for your team or organization, chances are many of them can be deleted once your development branch has been merged. Why not choose to delete those packages once your MR has been merged successfully?

Moving forward, you can now simply check a box in your MR and any packages that you published from your development branch will be automatically deleted and removed from storage. This will allow you to save money on storage and make it easier to find valid packages in the UI.

Problem to solve

While working on a feature branch, a Developer will often publish versions of a given package that, once their branch has been merged, do not serve any value. But, there is no easy way to do this today in GitLab. You would have to delete them from the user interface by searching and then deleting them. Or, you'd have to use the Packages API.

Proposal

Give Developers an easy way to delete their packages once their branch has been merged. Similar to #26983, allow users to choose to delete these packages once their MR has been successfully merged.