Resolve cross join issues in ee/lib/ee/gitlab/background_migration/migrate_shared_vulnerability_scanners.rb

The mentioned file contains cross join permissions as a result of the Sec DB decomposition. These need to be removed to ensure clean code quality and avoid hiding any cross join issues in the future unintentionally.

Edited by Gregory Havenga (is on PTO from 15 December 2025 to 02 January 2026)