Resolve vulnerability cross transaction issues in `ee/app/services/security/ingestion/ingest_slice_base_service.rb`

Problem to solve

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

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

Implementation

This service inserts records across many vulnerability tables during report ingestion. We have allowed transactions to across gitlab_main and gitlab_sec so we don't need to decompose foreign keys between vulnerability tables. Once all tables are migrated to the gitlab_sec DB we should remove the nested transactions since we can now rely off a single DB and connection to perform the inserts.