Add migration to create index on vulnerability reads

What does this MR do and why?

We need to be able to find all vulnerabilities for a project that are on the default branch and were detected by a specific scanner. To accomplish that, this MR introduces a sync migration to create a composite database index on vulnerability_reads(project_id, scanner_id, vulnerability_id). The work in this MR is based on the work merged in !110700 (merged).

SELECT "vulnerabilities".*
FROM "vulnerabilities"
INNER JOIN "vulnerability_reads" "vulnerability_read"
  ON "vulnerability_read"."vulnerability_id" = "vulnerabilities"."id"
WHERE "vulnerability_read"."project_id" = 1216
  AND "vulnerability_read"."scanner_id" = 489;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/15140/commands/52583

Additional context can be found in #382625 (closed).

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