GraphQL: Add maintenance_note_html field

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds the maintenance_note_html field to RunnerType, so that Markdown content on maintenance_note can be rendered through maintenance_note_html. It will be used notably in Add maintenance notes in runner view (!88060 - merged) to render markdown content in the maintainer_note field.

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How to set up and validate locally

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

Query (choose a ci_runner model that contains a maintenance_note value - can be changed in the PostgreSQL console gdk psql):

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

Output:

{
  "data": {
    "runner": {
      "id": "gid://gitlab/Ci::Runner/1206",
      "description": "Group test runner",
      "version": "14.11.0~beta.29.gd0c550e3",
      "maintenanceNote": "<h2>Test</h2>",
      "maintenanceNoteHtml": "<h2 dir=\"auto\">&#x000A;<a id=\"user-content-test\" class=\"anchor\" href=\"#test\" aria-hidden=\"true\"></a>Test</h2>"
    }
  }
}

MR acceptance checklist

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