Add a landscape cleanup action

@popey has been doing some great work pruning busted URLs and busted logos via scripts. We've been chatting about this at kubecon so writing it down. This is a tracking issue for taking those scripts and moving them to a github action.

- Then we can cron cleanups in the background
- We can clean up other cruft, etc.
- Reuse this across the landscape of landscapes

Start by locating the existing scripts that prune busted URLs and logos, since the issue does not name their files or tests. Clarify which cleanup tasks belong in the action, how cron scheduling and reuse across landscapes should work, and what successful cleanup looks like before implementation. Done means those scripts run as a reusable scheduled GitHub Action.

Written by the indexing model from the issue text.