GraphQL: Merge Request add field "Closed At"

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What does this MR do and why?

Adds field to GraphQL Endpoint for merge requests as requested in #424876 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. In local GDK navigate to http://127.0.0.1:3000/-/graphql-explorer
2. Set MR http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/merge_requests/1 to closed
3. run GraphQL query

{
  project(fullPath: "gitlab-org/gitlab-shell") {
    mergeRequest(iid: "1") {
      mergedAt
      createdAt
      closedAt
    }
  }
}

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