GitLab 16.0 requires instance admins to have permission to push protected branches

On deployments in which ghostflow-director is configured to use an instance admin user to push, Do: merge no longer works with protected branches unless the admin user is explicitly added as a member of the target project. This is an intentional change by GitLab:

• https://docs.gitlab.com/ee/update/removals.html#gitlab-administrators-must-have-permission-to-modify-protected-branches-or-tags
• https://gitlab.com/gitlab-org/gitlab/-/issues/12776

We need to decide how to deal with this in ghostflow-director deployments.