Transition sbom_occurrences to loose foreign keys

Proposal

Once we have support for a dedicated sec DB we need to transition the first phase models (sbom_*) to loose foreign keys to allow us to replicate the tables to the new DB and switchover seamlessly

Implementation Plan

• Update base class for Sbom::Occurrence 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, ❌ sbom_*), 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