Migrate to main as the default branch name

Created by: LawnGnome

This brings src-cli into line with our other important repositories. I don't believe we have any other hardcoded references to src-cli's master branch across our organisation, based on this search. There now is: https://github.com/sourcegraph/sourcegraph/pull/13228 will fix that, once this is merged.

Process wise, here's how I intend to proceed after approval:

1. Merge this PR.
2. Push master to main.
3. Update the default branch in the repo settings to main.
4. Edit open PRs to be against main instead of master.

If I've missed anything, I'd definitely appreciate a heads up! 😃