Change usage of StuckImportJobsWorker::IMPORT_JOBS_EXPIRATION to Gitlab::Import::StuckImportJob::IMPORT_JOBS_EXPIRATION

This issue is a follow-up for !32643 (merged)

During the refactoring of StuckImportJobsWorker I have avoided to also replace usages of StuckImportJobsWorker::IMPORT_JOBS_EXPIRATION to aGitlab::Import::StuckImportJob::IMPORT_JOBS_EXPIRATION` as it would have created a much bigger and harder to review MR. It is also a good candidate to move to a much smaller MR and easier to reviewe