Issue title: Start moving repositories master branch to main
 Issue body: There are a number of repositories that we can migrate to use main right away:

* cocoon
* recipes

Migrating those two repos will give us enough information about what will be required to migrate repositories with more traffic like engine. Flutter/flutter has its own challenges and will require its own design doc.
 Comments: 
 Comment 0: GoB configurations to keep master and main in sync have already landed.

https://flutter-review.googlesource.com/c/infra/+/18342 to move recipes to use main.
 Comment 1: CL to use main branch in engine builders: https://flutter-review.googlesource.com/c/infra/+/18344
 Comment 2: Start using main branch from cocoon repository: https://flutter-review.googlesource.com/c/recipes/+/18361
 Comment 3: @godofredoc If you want to set up `master`->`main` mirroring for flutter/plugins and flutter/packages, I can start updating scripts and CI jobs as I have time; I would expect that they will be fairly straightforward repos to migrate.
 Comment 4: Thanks @stuartmorgan, I'll send the changes for review during the day.
 Comment 5: I think `plugins` and `packages` are likely ready to flip the mirroring and lock `master`, but I'm not actually sure how to test all the Cirrus jobs on `main`. Is there a playbook for that?
 Comment 6: There is no playbook for cirrus that I'm aware of but it seems like we only need to change this line: https://github.com/flutter/plugins/blob/master/.cirrus.yml#L12
 Comment 7: Please let us know when you would like to finalize the migration to block some time on the engprod calendars to help with anything that may arise.
 Comment 8: > Please let us know when you would like to finalize the migration to block some time on the engprod calendars to help with anything that may arise.

If there's enough time next week we could do `packages` at the beginning of the week (so there's time to roll it back if necessary) as a small-scale test of the CI and tooling--most of which is very similar to that of `plugins`. Then if everything goes smoothly we can do `plugins` when everyone is back in January.

If not, let's plan on `packages` early in January, followed by `plugins` shortly after.
 Comment 9: Seems like `packages` went smoothly; can we schedule `plugins` for the first week of January?
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