Implement WorkItemWidgetDevelopment.relatedMergeRequests field

What does this MR do and why?

The field was previously introduced with an empty implementation. Now the field is implemented to actually return a list of related merge requests based on system notes.

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

Use GraphiQL to execute a query locally at http://localhost:3000/-/graphql-explorer

{
  workItem(id: "gid://gitlab/WorkItem/<id>") {
    id
    widgets {
      type
      ... on WorkItemWidgetDevelopment {
        relatedMergeRequests {
          nodes {
            id
            title
          }
        }
      }
    }
  }
}

Related to #503834 (closed)