GraphQL: Add maintenance_note to RunnerType

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds the maintenance_note field to the GraphQL RunnerType.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit http://gdk.test:3000/-/graphql-explorer (with an existing runner registered)

2. Enter the following GraphQL query:

   {
     runner(id: "gid://gitlab/Ci::Runner/1206") {
       id
       description
       version
       maintenanceNote
     }
   }

The result should include the new field:

{
  "data": {
    "runner": {
      "id": "gid://gitlab/Ci::Runner/1206",
      "description": "Group test runner",
      "version": "14.11.0~beta.29.gd0c550e3",
      "maintenanceNote": "Test maintenance note"
    }
  }
}

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.

Part of #348299 (closed)