Rename StuckImportWorker to StaleImportWorker

What does this MR do and why?

Rename StuckImportWorker to StaleImportWorker

We leave the old queue/worker class in place to be safely removed at a later date after no more jobs are scheduled.

My initial run at this included migrating the queue, but given that this is a single (idempotent) job that triggers once every four hours, I don't think it's worth it.

In the worst case scenario, the job triggers twice (but it's idempotent so that's fine) or misses an iteration and triggers after 8 hours instead of four hours, which should also be fine.

Changelog: other

Related issue: #434956 (closed)

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