Use MergeRequests::V2ApprovalRules::CreateService for CREATE requests when v2 feature flag is enabled. Requirements: - Update EE::API::MergeRequestApprovalRules POST endpoint to check use_v2_approval_rules? flag - Call v2 CreateService when enabled, v1 service when disabled - Add API request specs testing success, validation failures, authorization failures, and feature flag toggle - Ensure response format matches v1 Reference: v2 Approval Rules POC (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/222865)