🛠 Repo: Rename master branch to main

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest master branch of the repository.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Per https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main, the default branch for repositories is now main. We're going to apply the same change to this repo and its tooling. Filing this issue for tracking.

Additional Info

https://github.com/github/renaming has more details on the context and practices around the rename.

This'll need a find-and-replace in any docs, workflows, etc. that refer to branches/Git/etc.

Intentional follow-up to #5038.

[JoshuaKGoldberg]

We'll do this, but need to wait until we have website access and are a bit more experienced as maintainers. Likely to wait until at least one release from master is done.

[JoshuaKGoldberg]

We have website access now (mostly), and shoutout to @voxpelli for running the v10.4.0 release. Marking as unblocked!