Move merge request approvals API code to EE
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=408183) </details> <!--IssueSummary end--> ### Problem According merge request approvals [API documentation](https://docs.gitlab.com/ee/api/merge_request_approvals), this functionality was moved to Premium. However, [the code was not migrated under EE scope](https://gitlab.com/gitlab-org/gitlab/blob/453da309da660bf3bb3b83c76ca9ded3b00c905a/lib/api/merge_request_approvals.rb). ### Proposal * Merge `API::MergeRequestApprovals` into `EE::API::MergeRequestApprovals`. * Verify [documentation for merege request approval responses](https://docs.gitlab.com/ee/api/merge_request_approvals) as it can be incorrect
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