Expose dismissal_reason on VulnerabilityType

What does this MR do and why?

Expose dismissal_reason on VulnerabilityType

Related to #411774 (closed)

How to set up and validate locally

GraphQL query to use

query {
  project(fullPath: "<path to project>") {
    vulnerabilities {
      edges {
        node {
          id
          state
          dismissalReason
        }
      }
    }
  }
}

1. Ensure you have a project with some Vulnerabilities present in the Vulnerability report
2. Dismiss any of the Vulnerabilities
3. Execute the provided query, dismissalReason should be null
4. echo "Feature.enable(:expose_dismissal_reason)" | rails c
5. Execute the provided query, dismissalReason should NOT be null for the dismissed Vulnerability

Database review

This change adds preloading vulnerability_reads table:

Query plan (cold cache): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65032

Query plan (warm cache): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65033

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.