Rename master branch to main branch

[rniwa]

Per https://www.w3.org/Guide/manual-of-style/#inclusive, we should rename master branch to main branch.

Also see w3c/idcg#17

[lucacasonato]

xref web-platform-tests/rfcs#80

[foolip]

I'd also like to see this happen. If someone would like to take over web-platform-tests/rfcs#80 and drive it to completion, I would be very happy. It's a lot of work to ensure this change won't break anything given all the repos involved, and also things like https://wpt.fyi/results/?label=experimental&label=master&aligned where we'd need some changes in wpt.fyi.

[jgraham]

Yes. This is not a trivial change since there's quite a lot of infrastructure to update, spread across multiple repositories and consumers of wpt. Despite that, I agree that it's a good idea if we can come up with a plan that isn't just "break consumers and hope they update". However I'm going to close this issue to ensure that discussion happens in one place (the RFC).