Rename and protect the default branch

[Jyyjy]

1. Rename the master branch to main so that this repo matches current defaults and is more welcoming to new contributors.
2. Add branch protection rules to main that require a pull request with an approval to commit.
3. Update GitHub actions to use main instead of master.
4. Notify dev list to update their locals as such: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch#updating-a-local-clone-after-a-branch-name-changes

Steps 1 and 2 require an Apache infra ticket.