reusable-github-workflows v0.39.0 — mirror/archive skip dependabot

Dependabot runs get no repository secrets, so the mirror and archive jobs failed
on every dependency bump. Every job in git-mirror.yml and archive.yml now skips
when the actor is dependabot or the branch is a dependabot/ branch.

This was the only remaining source of red on dependabot branches across the
fleet; the test pipelines already pass there. Callers ride @master, so no caller
edit is needed.

Also corrects the README's max_new_items_per_day default from 10 to 5.