Clean up expired Direct Transfer exports
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This is a follow-up issue from https://gitlab.com/gitlab-org/gitlab/-/issues/408469#note_1930624460
> It just means we start to export more things on disk, so we probably need a cron job for direct transfer exports to clean up old exports. **We do the cleanup for file-based, but I don't think we do for DT.** (emphasis mine)
The exports associated with file-based exports are pruned in `Gitlab::Export::PruneProjectExportJobsWorker` & `Projects::ImportExport::PruneExpiredExportJobsService`.
We need to either update these create equivalent workers & cron schedule to handle exports generated by Direct Transfer.
I think the key logic is:
```ruby
BulkImports::Export.where(created_at: ..1.week.ago).each(&:remove_existing_upload!)
```
* The existing logic for starting a new export removes the attached file, but keeps the `BulkImports::Export` record itself.
* 7 days matches the expiry defined in `ProjectExportJob`
issue