Synchronous creation: index_vuln_findings_on_uuid_including_vuln_id

This index was originally added in an async migration in the following
two MRs:

1. [Index vulnerability findings on uuid including vuln id][0]
2. [Recreate async index `index_vuln_findings_on_uuid_including_vuln_id`][1]

This commit is a followup to add the index synchronously as the
final step in the [Asynchronous Index Creation][2] process

[0]:https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115175
[1]:https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125646
[2]:https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#create-indexes-asynchronously

Changelog: performance