Add an application setting to set max number of running jobs for MergeRequestCleanupRefsWorker

Problem to solve

In #296874 (closed), we update MergeRequestCleanupRefsWorker to be a limited capacity worker. For first iteration, it's been set to 4 at a time so we can test.

That may be too low though and we may have capacity to have a higher concurrency especially on .com. But at the same time, we also need to think about self-managed installs.

Proposal

Add an application setting that admins can set which will be the max_running_jobs of the MergeRequestCleanupRefsWorker.