Finalize `PurgeSecurityScansWithEmptyFindingData` background migration

Why are we doing this work

In %16.10, we have introduced a background migration to purge security_scans records associated with legacy security_findings. We need to finalize that migration by %16.11.

Relevant links

• The MR introduced the background migration: !144456 (merged)

Implementation plan

• database Create a post-deployment migration to finalize PurgeSecurityScansWithEmptyFindingData

Verification steps

• Make sure the migration succeeds