GitHub Action to delete merged stale branches

[OlufunkeMoronfolu]

This PR introduces a GitHub Action that automates branch deletion and removes the need for authors to delete branches by themselves (as we currently do).

This Loop document shares more info about the process involved with creating this PR.

Activity diagram for easy understanding:

[ConnorLand]

I am a little wary of monthly automated branch deletions, as it is possible that I might merge something and then need to go back to the merged branch 4 weeks later for something.

For safety, would it be possible to set PRs assigned to me as Manual delete status only?

[ConnorLand]

• Branches with "backup" in the name are never deleted, regardless of their merge age or whether they have been merged into the development branch or not

I think Olu's writing above solves my issue. I'll just include "backup" in the important ones.

[MarkvanMents]

Hi @OlufunkeMoronfolu
This all looks good as far as I can tell.
However, it might be a good idea for you to walk me through the code so that I can understand it better and we can ensure that there are no edge cases we have forgotten.

But it is a great use of GitHub actions and perhaps will help us to create more actions in the future?

Set up a meeting when you are back from PTO and we can go through it together - perhaps when we are both in the office.

Thanks for doing this

Mark