Deprecation of the `merged_by` API field

[fbpe]

Gitlab 14.7 deprecates the merged_by field in their merge request API - planned removal is for Gitlab 16.0 (2023-05-22): https://docs.gitlab.com/ee/update/deprecations.html#merged_by-api-field

They introduced a new field merge_user to replace it, so it should probably be used instead, deprecating the existing merged_by.