Adding more loose foreign keys for ci_pipelines

These [loose foreign
keys](https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html)
are the first step to removing the cross-database foreign keys between
`main` and `ci` databases.

These 3 foreign keys have been determined to be safe for swapping to
loose foreign keys per
https://gitlab.com/gitlab-org/gitlab/-/issues/348269 .

These Loose foreign keys do not require any tests yet until we actually
remove the foreign key as we can't validate the behaviour if the real
foreign key already does the cleanup for us and we already have a test
to show that deletes to `ci_pipelines` are tracked.

lib/gitlab/database/gitlab_loose_foreign_keys.yml

@@ -43,6 +43,18 @@ ci_project_mirrors:
   - table: namespaces
     column: namespace_id
     on_delete: async_delete
+merge_requests:
+  - table: ci_pipelines
+    column: head_pipeline_id
+    on_delete: async_nullify
+vulnerability_statistics:
+  - table: ci_pipelines
+    column: latest_pipeline_id
+    on_delete: async_nullify
+vulnerability_occurrence_pipelines:
+  - table: ci_pipelines
+    column: pipeline_id
+    on_delete: async_delete
 packages_build_infos:
   - table: ci_pipelines
     column: pipeline_id