Use v2 approval rules for DELETE endpoint behind feature flag

Use MergeRequests::V2ApprovalRules::DestroyService for DELETE requests when v2 feature flag is enabled.

Requirements:

• Update EE::API::MergeRequestApprovalRules DELETE endpoint to check use_v2_approval_rules? flag
• Call v2 DestroyService when enabled, v1 service when disabled
• Add API request specs testing successful deletion, authorization failures, 404 errors, and feature flag toggle
• Ensure response format matches v1

Reference: v2 Approval Rules POC (!222865)