Database 2/2: Add hasMergeRequest filter to VulnerabilityReport

Issue to track the database implementation work to add has_merge_request filter to VulnerabilityReport.

So that the following CQL is supported see: #371313 (closed)

query {
  project(fullPath: "gitlab-org/gitlab") {
    vulnerabilities(reportType:DEPENDENCY_SCANNING, hasMergeRequest: true) {
      nodes {
        hasMergeRequest
      }
    }
  }
}

• database MR 3: Update vulnerability_reads trigger to set has_merge_request
  • Use the update trigger for has_issues !112233 (merged) as example.
• database MR 4: Backfill has_merge_request column with background migration

MR 1 & MR 2 work is tracked in #420613 (closed)

Non-functional requirements

• Performance: Need to see how this would affect the performance of vulnerability_reads
• Testing: New tests will need to be added
• E2E testing: Make sure e2e: package-and-test is run and govern specs are green