Add GraphQL migration to set epic of an issue

What does this MR do?

#36309 (closed)

Add the GraphQL migration to set the epic of an issue

mutation {
  epicAddIssue(input: {groupPath: "gitlab-org", projectPath: "gitlab-org/test-open-project", iid: "5", issueIid: "1"}) {
    epicIssue {
      id
      title
      epic {
        id
        title
      }
    }
    errors
  }
}

Example response

{
  "data": {
    "epicAddIssue": {
      "epicIssue": {
        "id": "gid://gitlab/Issue/454",
        "title": "Test Issue 1",
        "epic": {
          "id": "gid://gitlab/Epic/5",
          "title": "Voluptatem voluptatem aut tenetur sequi rerum voluptatibus deserunt."
        }
      },
      "errors": []
    }
  }
}

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #36309 (closed)