Geo: Add a backoff time to few more workers

What does this MR do?

Add a backoff time to some Geo workers where it's reasonable.

The following workers were selected:

• Geo::MigratedLocalFilesCleanUpWorker
• Geo::FileDownloadDispatchWorker
• Geo::RepositoryShardSyncWorker
• Geo::RepositoryVerification::Secondary::ShardWorker
• Geo::RepositoryVerification::Primary::ShardWorker

What are the relevant issue numbers?

#7593 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7593 (closed)