renaming the default branch from `master` to `main`

[devoncarew]

We're renaming the default branch here from master to main. If you have a local checkout, after the rename, you should run:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

(the github UI will prompt you to run that when you first revisit the page after the rename)