Replace `master` branch with `main`

[clcollins]

We've started to make changes to replace non-inclusive terms in other parts of the infrastructure, including "blacklist" and "whitelist". We should make the same change to the terminology of the "master" branches, and since this is a young repo with little hard-coded references to branch names, the time is perfect to make this change here as well.

Please investigate and replace the master branch with main as the primary/default branch, and remove master completely.

As part of this, please document how you complete this, and any issues that arise so we can share this with others in the team.