Backfill vulnerabilities.finding_id column

What does this MR do and why?

This MR schedules a batched background migration to backfill the vulnerabilities.finding_id column in attempt to create a proper 1:1 relationship between records in vulnerabilities and vulnerability_occurrences tables.

See &11030 (closed) for a high level overview

Related to #418971 (closed)

Database review

Query plan

UPDATE vulnerabilities
SET finding_id = vulnerability_occurrences.id
FROM vulnerability_occurrences
WHERE vulnerabilities.id IN (<100 ids here>)
AND vulnerabilities.id = vulnerability_occurrences.vulnerability_id

Cold cache: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/21644/commands/70316

Warm cache: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/21644/commands/70317

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.