Switch from master to main for primary branch

[nzakas]

• GitHub: Merge any outstanding pull requests
• Local: Ensure you're up-to-date locally git fetch && git reset --hard origin/master
• Local: Push a main branch: git checkout -b main && git push origin main
• GitHub: Under Settings -> Branches: Set the default branch to main
• GitHub: Under Settings -> Branches: Set up branch protection for the main branch
• GitHub: Under Settings -> Branches: Remove branch protection for master
• Local: Delete remote master branch git push origin :master
• Local: Delete master branch git branch -d master