Add maintenance note field when editing projects and group runners

Description

Group owners and project maintainers can edit maintenance notes on the API side for runners in groups and projects. They are not able to in the UI today.

More details

Our runner maintenance note is only visible to users in the admin section and we should expand the usage to more user roles. As @tmaczukin said:

The whole idea of my proposal for this note was that in gitlab-org group we have few private runners that SaaS Runner group owns and I've already seen few "unauthorized" configuration changes like extending the maximum timeout. And I want this note to be present there for all gitlab-org group owners and maintainers to clearly state that this runner should not be edited without consultation with Runner SaaS group 🙂

Proposal

Add a maintenance note field when editing group runners and project runners if the user has edit permissions (admins, group owners, or project maintainers).

Technical notes

Currently we have a site-wide runner_maintenance_note EE feature, we could add this to runner_maintenance_note_for_namespace variant that gates the feature, GitLab Ultimate users should then get access to it.