Make concurrency of Ci::DeleteObjectsWorker dynamic

As part of the follow-up to !207715 (comment 2807701325)

We want to change max_running_jobs to read from ApplicationSetting instead of a hardcoded number.

There is a little bit of work here because instead of adding another column specific to our use case, we want to add a jsonb column that can be re-used by multiple sidekiq workers to input their own concurrency threshold