Deprecation of the `merge_status` API field

[fbpe]

Gitlab 15.6 deprecates the merge_status field in their merge request API - planned removal is for Gitlab 16.0 (2023-05-22): https://docs.gitlab.com/ee/update/deprecations.html#merge_status-api-field

They are introducing a new field detailed_merge_status to replace it, so it should probably be used instead, deprecating the existing merge_status.