Support determining activity date from default branch

[anovadox]

Pull Request

New Staleness criteria: latest default_branch commit

Proposed Changes

Support determining activity date from default branch rather than most recent push to any branch

ACTIVITY_METHOD defaults to the current behavior: repo.pushed_at aka "pushed"

However, I think the most recent commit to the default branch (default_branch_updated) is another valid way of determining staleness/activity.

Our organization uses dependabot, which continues to open PRs and push commits to repositories which have not had any human contribution in more than a year.

With this change, we will easily be able to surface repositories that otherwise would be missed due to bot activity keeping them "fresh".

Readiness Checklist

Author/Contributor

• [x] If documentation is needed for this change, has that been included in this pull request
• [x] run make lint and fix any issues that you have introduced
• [x] run make test and ensure you have test coverage for the lines you are introducing

Reviewer

• [x] Label as either bug, documentation, enhancement, infrastructure, or breaking

[zkoppert]

Thanks for adding this enhancement @anovadox! Appreciate you adding this!