Rename default branch from "master" to "main"

[hugovk]

To match most of the other https://github.com/python repos, let's use main as the default branch.

Please could @python/organization-owners rename it?

Docs here:

• https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch

[CAM-Gerlach]

While they're at it, could they also complete the other two items from #460 that are still waiting for an org admin's intervention? I've asked around and I couldn't find anyone who was a repo admin here specifically.

• Enable tag protection for *
• Rename master to main
• Add a branch protection rule for * with the settings in this screenshot:

And maybe those mentioned in the linked comment in Bedevere too (or at least all but the last step)?

[hugovk]

(Converting to draft until the branch is updated.)