Automate clean up of warden registration branches

[jacobheun]

Problem / opportunity:

There currently is no clear retention policy on warden registration PR branches. When closed, the branches aren't always deleted, leaving an excessive number of branches remaining in git history.

Desired outcome

Branches for closed PR's are automatically deleted after 30days.

Solution

Add a github action to automatically clean up PR branches that have been closed after 30 days.

Additional context

Github PR branch retention period is currently indefinite, so branches can be manually deleted on closed PR's right away. We shouldn't rely on a human needing to do this though. 30days is probably excessive, in reality this could probably be immediate, but we do need to bulk remove the older branches if we did this as a PR closed action.