Resolve Vulnerabilities::ManuallyCreateService cross transaction issues

Problem to solve

All cross-joins arising from decomposing gitlab_sec from gitlab_main should be resolved. See parent epic for context.

1. Remove allowed cross-join for relevant relation using associated documentation for guidance on removing cross-joins or cross-transactions occurrences

In the case of ManuallyCreateService, we allow-listed cross-transactions (example). We should resolve these once the sibling tables are also migrated, in this case once all vulnerabilities tables co-exist with vulnerability_scanners