Prepare index for optimized searches from the vulnerability_reads table

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 post migration to begin generation of a composite database index on vulnerability_reads(project_id, scanner_id, vulnerability_id). Additional context can be found in #382625 (comment 1259314425).

This MR follows the guidance found in Database development guidance.

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

MR acceptance checklist

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