api/runners: Expose `maintenance_note` in details and editing

What does this MR do and why?

api/runners: Expose maintenance_note in details and editing

Related to: #422621 Fixes: #438199 Changelog: changed

MR acceptance checklist

Didn't see where to add new tests for this.

Screenshots or screen recordings

N/A.

How to set up and validate locally

No idea; I've never gotten an GitLab development env setup. Trusting CI…