Return Empty approvers and approver_groups from project approval API and project and merge_request presenters

MR - Return empty array for approver and approver_gr... (!206807 - merged)

Description

We have the project approval API marked approvers and approver_groups fields as deprecated, and should always return empty array, but we are still returning values on those two fields. Add feature flag on entity to return empty array.

Those values are being populated as Project and MergeRequest presenters, and we would like to add a feature flag and return empty array at this place as well.

https://docs.gitlab.com/api/merge_request_approvals/#approval-rules-for-projects