Do not allow maintainers to push to protected branches directly

Spinning off from https://gitlab.com/gitlab-jh/gitlab/-/issues/85#note_602116587 where I accidentally pushed to main-jh which should be disallowed. I am revisiting who can merge and push for protected branches.

I am setting only @gitlab-jh-bot can push to master because this branch should be read-only, and only @gitlab-jh-bot should be able to push to it in order to mirror the master branch from EE.

And only @project_23966655_bot2 can push to main-jh for the code synchronization. Maintainers should be able to merge, of course.