Convert master to main

[coke]

Following the lead of other Raku and Rakudo repositories, we should switch this repo's primary branch from master to main.

I recommend

1. copying the branch just after a Rakudo release
2. updating all the tooling in Rakudo to cope (make spectest, make release, etc.),
3. protecting the old master branch to avoid new commits
4. Announce the change for anyone who needs to track it manually
5. Setting a deadline a few releases out, and then removing it.