Enable auto-merge for all repos

[kdmccormick]

The PR auto-merge feature is useful and we've gotten at least one request from maintainers for it to be turned on for their repo. We've been turning it on for Axim-maintained repos too. We think it'd be beneficial for the community if it were available on every repo.

AC

• add a check to repo_checks.py (in terraform-github) that will enable it across all repositories
• include a brief ADR for the .github repo
• cross-post the ADR to the maintainer section of the forums

[feanil]

This was done via https://github.com/openedx/repo-tools/pull/494