Chore: stale branch cleanup

[dgkf-roche]

Noticed we have a lot of lingering branches for merged PRs still cluttering our repo.

@maksymiuks wanted to check in with you before I clear out any old ones.

[maksymiuks]

I forgot GitHub does not have a "Remove after merging" option like GitLab does (or does it?).

In any case, except for the 37-a-rev-dep-check-that-mimics-a-cran-like-environment, I don't use any of the branches, and as far as I'm concerned, they can be removed (well, except for the main :D)