Deprecate MergeRequest#mergeStatus

[marcelstoer]

Fixes #880

[jmini]

IMO this change should update the corresponding test:
https://github.com/gitlab4j/gitlab4j-api/blob/master/src/test/resources/org/gitlab4j/api/merge-request.json

By adding a line like:

  "detailed_merge_status": "can_be_merged",

See doc: https://docs.gitlab.com/ee/api/merge_requests.html#response

[jmini]

Looks good to me