Rename default branch

[andymckay]

👋 This issue is to track the move over to using main as the default branch for this repo. We’d love your team's help in completing this transition.

Do not remove your old default branch, customers are going to be using it. We will be sending messages out about these changes, but if you want to message in your repository, that's fine as well.

• [x] Create a main branch.
• [x] You might need to rebase any pull requests you want to merge before changing the default branch.
• [x] Change the default branch in settings to main.
• [x] Update any documentation in this repo to refer to the new branch name, although using the version tag is still preferred.
• [x] Check that this Action works correctly for users who have a repository with a custom default branch name.
• [x] Close this issue and celebrate 🎉

We are aiming to complete this work by ~July 17th~ August 28th.

[andymckay]

Looks like there's no documentation that references the master branch or anything in the code that cares what branch the users repo is on.