Remove existing orphaned Upload records for BulkImports::ExportUpload

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Follow-up to #542228 (closed) - now that the MR has been deployed and prevents new orphaned Upload records from being created, we need to clean up the existing orphaned records that were left behind.

This should involve:

• Identifying all orphaned Upload records where the associated BulkImports::ExportUpload record no longer exists
• Creating a migration or cleanup script to safely remove these orphaned records
• Verifying the cleanup resolves the Geo verification errors

The orphaned records were previously identified in the Geo verification errors on staging-ref.gitlab.com.