GraphQL: Add maintenance_note to UpdateRunner mutation

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 UpdateRunner mutation.

Screenshots or screen recordings

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scenario	screenshot
Successful update
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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:

   mutation runnerUpdate($input: RunnerUpdateInput!) {
     runnerUpdate(input: $input) {
       runner {
         maintenanceNote
       }
       errors
     }
   }

3. Enter the JSON query variables with the ID of the runner to modify:

   {
     "input": {
           "id": "gid://gitlab/Ci::Runner/1",
           "maintenanceNote": "Test"
           }
   }

The result should include the new field:

```json
{
  "input": {
        "id": "gid://gitlab/Ci::Runner/1",
        "maintenanceNote": "Test"
        }
}
```

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)