Refactor existing Gitlab required reviewers option to support latest API implementation

[dkichler]

The existing Gitlab required reveiwers feature is based on a Gitlab API endpoint that has been deprecated and will be removed in a future release: https://docs.gitlab.com/ee/api/merge_request_approvals.html#change-approval-configuration-deprecated

Refactor this option/feature to use the replacement API from Gitlab to achieve the same effect. (https://docs.gitlab.com/ee/api/merge_request_approvals.html#create-merge-request-level-rule)

[endertunc]

I can pick this up if no one stared already.