Use v2 approval rules for UPDATE endpoint behind feature flag

Use MergeRequests::V2ApprovalRules::UpdateService for UPDATE requests when v2 feature flag is enabled.

Requirements:

• Update EE::API::MergeRequestApprovalRules PUT/PATCH endpoint to check use_v2_approval_rules? flag
• Call v2 UpdateService when enabled, v1 service when disabled
• Add API request specs testing updates to name, approvals_required, user_ids, group_ids, remove_hidden_groups
• Test validation failures, authorization failures, and feature flag toggle
• Ensure response format matches v1

Reference: v2 Approval Rules POC (!222865)