`master` → `main` branch renaming

Calling all maintainers of both clients and tldr alike!

The master branch has now been renamed to main.

Client authors

While a new mirror branch master that reflects main has been created, please update your clients if required to default to main instead of master. In the future the master mirror branch will be deleted.

Note though that it is RECOMMENDED that you download our tldr.zip instead of cloning this repository, as per our client specification if appropriate.

tldr maintainers

All existing PRs and branch protection rules have been updated.

Please watch out for new PRs that merge against master, as I have been unable to create a branch protection rule on master that doesn't also block GitHub Actions from pushing to it.