Backfill vulnerability risk_score in ES index
Relates to issue #571078 (closed)
This MR was created by Duo in Session 1202984
Changes
- Create Elasticsearch migration to backfill risk_score field for existing vulnerability documents
- Use MigrationReindexBasedOnSchemaVersion helper for efficient reindexing
- Apply skip_if directive to run only on GitLab SaaS and Dedicated deployments
- Ensure risk_score field is populated from vulnerability_finding_risk_scores table