Transition security_findings to loose foreign keys

Implementation Plan

• Update base class for Security::Findings to rely on Gitlab::Database::SecApplicationRecord
• Update db/docs/TABLE_NAME.yml to point to new gitlab_sec schema
• If table references tables beyond the sec DB boundary (i.e. ✅ projects, ❌ security_*), use the generate-loose-foreign-key script to decompose foreign keys for target table
• do while verifying behavior
  • Fix cross-database transactions
  • Fix cross joins