Rename Runner model and API maintainer_note field to maintenance_note

Summary

Based on the discussion in gitlab-runner!3268 (comment 815663628) it emerged that the new maintainer_note field name (also released in a REST API in %14.7 through !77779 (merged)) was not a good name for several reasons, namely because 'maintainer' is the name of a role, which does not necessarily need to match the role in which this field is used, and there is already an instance of a maintainer_note in our code which is related to the Note model.

Implementation

1. Create an alias in the Ci::Runner model;
2. Accept the new maintenance_note field in the REST API in addition to maintainer_note. Convert maintainer_note to maintenance_note upon reception;
3. Mark the maintenance_note field in the REST API as deprecated.

Links

• gitlab-foss#47592 (comment 80050827)
• follow-up Rename ci_runners.maintainer_note -> ci_runners... (#351089)