Rename branch `master` to `main`

[csarven]

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[steve-bate]

+1 although I think this will break any existing forked repositories.

[evanp]

We're following best practices for Git repositories to take the divisive term master out of the default branch, and instead use main. GitHub describes how this has worked for other repositories here:

https://github.com/github/renaming

This is the default for new repositories, and over time systems will be depending on the default branch being main, so we should catch up.

For those with a clone of the AS2 repo, you'll need to do these steps to upgrade:

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