Use v2 approval rules for CREATE endpoint behind feature flag

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 (!222865)