Use ExclusiveLeaseGuard module in StuckBuilds workers

What does this MR do and why?

This MR implements an exclusive lease for the StuckCiJobsWorker and Ci::StuckBuilds::DropRunningWorker using the common logic in ExclusiveLeaseGuard, instead of implementing our own inside the worker class. In addition to centralizing the logic itself, the specs are much cleaner since we don't need to test our re-rolled lease guards individually.

@dbalexandre pointed this out during maintainer review of the Ci::StuckBuilds::DropScheduledService, so I'm going back to clean these up in the same way.

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