idx_vuln_occurrences_on_partition_id_primary_identifier_id synchronous database index addition

Summary

This issue is to add a migration to create the idx_vuln_occurrences_on_partition_id_primary_identifier_id database index on vulnerability_occurrences synchronously after it has been created on GitLab.com.

The asynchronous index was introduced in !244112 (merged).

The synchronous migration is prepared in !244113 (merged) (Draft, blocked on the async MR; mark ready only after the index is confirmed created and valid on GitLab.com).

Related to #596858 (closed) Schema change exception request: https://gitlab.com/gitlab-org/database-team/team-tasks/-/work_items/656