Rename and protect the default branch

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.

Start by identifying the repository's GitHub Actions references to master and review the linked GitHub guidance for updating local clones. Coordinate the Apache infrastructure ticket for the branch rename and protection rules, then update the workflows and notify the dev list. Done means main is the default protected branch, Actions use main, and contributors have the local-update guidance.

Written by the indexing model from the issue text.