Switch from `master` to `main`?

[JanCVanB]

main is becoming the new default (already has on GitHub), and now seems like a good time, since we're already de-forking and revisiting our branching workflow.

I've already pushed a main branch at the master commit, so if we want to be cautious with this, we could

1. switch the "default branch" on GitHub (for cloning & PRs) to main
2. manually fast-forward master to meet main as PRs merge
3. slowly update production references from master to main
4. delete master when it feels safe

without needing to update all production references from master to main right away.