Protect loose foreign key dashboard
This change adds the loose foreign keys dashboard to the protected Grafana dashboards list.
- Reason: We still iterate on the dashboard, it's not final.
- Link: https://dashboards.gitlab.net/d/RKrvPFp7k/loose-foreign-keys
- Docs about loose foreign keys: https://docs.gitlab.com/ee/development/database/loose_foreign_keys