Add extra low concurrency flag for unlock pipelines worker

What does this MR do and why?

To help alleviate problems in #435051, we add a new ops flag to further decrease the max_running_jobs of Ci::UnlockPipelinesInQueueWorker to 10 as needed by smaller self-managed instances. Currently, our default is at 50, which may be too much for some.

I didn't want to change the default because this may suddenly affect other instances that are already running fine with the default of 50.