Deprecate(?) allow_maintainer_to_push option merge_request

I've discovered this while I was working on https://gitlab.com/gitlab-org/gitlab/-/issues/376941

It's marked as deprecated here for about 4 years.

It seems that it's an alias according to the API doc here https://docs.gitlab.com/ee/api/merge_requests.html#create-mr

I think we should check if this can be removed and remove if we can.