Early branch: Adjusts stable branch permissions on the release candidate

Context

When the initial release candidate is tagged, the ongoing stable branch for GitLab, Omnibus, and CNG, is open to maintainers so they can backport bug fixes. This automation is done via monthly_release_initial_rc:update_branch_permissions job

However, the automation only allows maintainers to merge, but it should also allow the gitlab-release-tools-bot and the gitlab-bot to push and merge. The purpose of this issue is to update the service to allow the bots to merge onto stable branches when the initial release candidate is tagged.