Use old_outstanding_balance on disabled toggle

No need to go through the `OrderBalance` in this context since we
already check for the toggle.

app/serializers/api/order_serializer.rb

@@ -11,7 +11,7 @@ module Api
       if OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, object.user)
         -object.balance_value
       else
-        object.outstanding_balance
+        object.old_outstanding_balance
       end
     end
 

engines/order_management/app/services/order_management/reports/bulk_coop/bulk_coop_report.rb

@@ -171,7 +171,7 @@ module OrderManagement
           if OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, @user)
             unique_orders(line_items).sum(&:new_outstanding_balance)
           else
-            unique_orders(line_items).sum(&:outstanding_balance)
+            unique_orders(line_items).sum(&:old_outstanding_balance)
           end
         end
 

engines/order_management/spec/services/order_management/reports/bulk_coop/bulk_coop_report_spec.rb

@@ -195,7 +195,7 @@ describe OrderManagement::Reports::BulkCoop::BulkCoopReport do
       end
 
       it 'calls #outstanding_balance' do
-        expect_any_instance_of(Spree::Order).to receive(:outstanding_balance)
+        expect_any_instance_of(Spree::Order).to receive(:old_outstanding_balance)
         subject.send(:customer_payments_amount_owed, [line_item])
       end
     end

spec/serializers/api/order_serializer_spec.rb

@@ -44,12 +44,12 @@ describe Api::OrderSerializer do
       end
     end
 
-    context 'when the customer_balance is not enabled' do
+    context 'when the customer_balance is disabled' do
       before do
         allow(OpenFoodNetwork::FeatureToggle)
           .to receive(:enabled?).with(:customer_balance, order.user) { false }
 
-        allow(order).to receive(:outstanding_balance).and_return(123.0)
+        allow(order).to receive(:old_outstanding_balance).and_return(123.0)
       end
 
       it 'calls #outstanding_balance on the object' do