Rename StuckImportWorker to StaleImportWorker

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/139676



Merged-by: Patrick Cyiza <jpcyiza@gitlab.com>
Approved-by: Martin Čavoj <mcavoj@gitlab.com>
Approved-by: Patrick Cyiza <jpcyiza@gitlab.com>
Reviewed-by: Patrick Cyiza <jpcyiza@gitlab.com>
Reviewed-by: Martin Čavoj <mcavoj@gitlab.com>
Co-authored-by: James Nutt <jnutt@gitlab.com>