Prepare async index for vulnerability_occurrences table

What does this MR do and why?

This new index will be utilized to address query timeout issues in override uuids logic.

Database review

The timing out query

SELECT
    "vulnerability_occurrences".*
FROM
    "vulnerability_occurrences"
WHERE
    "vulnerability_occurrences"."project_id" = 278964
    AND "vulnerability_occurrences"."report_type" = 0
    AND "vulnerability_occurrences"."location_fingerprint" IN (...)

• Before the index: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17493/commands/58516
• After the index: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17493/commands/58536

Related to OverrideUuidsService is raising `PG::QueryCance... (#397004 - closed).

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.