Enable shared runners in dev.gitlab.org mirror of omnibus-gitlab and move package promotion jobs to them

Continuation from gitlab-org/distribution/team-tasks#1297 (comment 1486428103)

The shared runners available in dev.gitlab.org also spins up n2d-standard-2 just like our dedicated package promotion runner. And package upload definitely doesn't require more resources than that.

Looking at the jobs in https://dev.gitlab.org/gitlab/gitlab-ee/ project where these runners are enabled, we can see that there is very trivial queue duration for the jobs as compared to the current situation where due to IdleCount being 1 in our current package promotion runner, the duration can go upto 6 minutes. While we can improve the situation with IdleCount, I can't find a very good reason to maintain a separate runner just for package upload jobs. We already have a managed runner fleet with sufficient resources for them, and we can just use them.

Proposal

Keep builds in Distribution owned runners (because we need beefier machines for builds), but move package promotion to shared runners.

For that, we need to

1. Backport !7048 (merged) to stable branches, so that jobs will stop using the old runner tags (which are present in shared runners also). We don't want builds happening in shared runners.
2. Enable shared runners in https://dev.gitlab.org/gitlab/omnibus-gitlab/-/settings/ci_cd.
3. Modify package promotion jobs to drop the promotion tag, so they can be picked up by shared runners.