chore: rename 'master' to 'main' in docs and tooling

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🛠 Repo: Rename master branch to main #5051
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Roughly a straightforward find-and-replace.

Once this is approved, I can rename the branches here and in github.com/mochajs/mocha-examples.

[coveralls]

coverage: 94.358%. remained the same
when pulling f3cf82a on JoshuaKGoldberg:master-to-main
into e263c7a on mochajs:master.

[voxpelli]

lgtm