Rename master branch to main

[FMJansen]

I noticed the main branch of this repository is still called master. I can see multiple repos, like https://github.com/GoogleChrome/web.dev/, have already switched to main. The chrome-devrel-review-bot also warns us to not use the word master, but that is difficult to do when lighthouse urls still contain that word.

I understand changing the name can have some unexpected impact, like when the devtools change required a code change in this repo (https://github.com/GoogleChrome/lighthouse/pull/13266) and because web.dev links to quite some lighthouse source code. Still, it seems like a good idea to move away from this kind of language and switch to calling it main.

[paulirish]

@FMJansen Thank you. We intend to do this, but have been slow to make it happen. But we shall.

[mel-ada]

@paulirish Wanted to check in because this is super important! Do you have a ballpark timeline? Are there technical blockers?

[paulirish]

Thanks ya'll. Apologies for the delay.