App configuration: Use SimpleOrderContents by default

With the null promotion configuration in place, we can't use the legacy
Spree::OrderContents class, as that makes references to the legacy
promotion system.

See #5755

core/lib/spree/app_configuration.rb

@@ -394,7 +394,7 @@ def default_pricing_options
     # @!attribute [rw] order_contents_class
     # @return [Class] a class with the same public interfaces as
     #   Spree::OrderContents.
-    class_name_attribute :order_contents_class, default: 'Spree::OrderContents'
+    class_name_attribute :order_contents_class, default: 'Spree::SimpleOrderContents'
 
     # Allows providing your own class for shipping an order.
     #

core/spec/lib/spree/app_configuration_spec.rb

@@ -36,6 +36,10 @@
     expect(prefs.variant_search_class).to eq Spree::Core::Search::Variant
   end
 
+  it "uses simple order contents class by default" do
+    expect(prefs.order_contents_class).to eq Spree::SimpleOrderContents
+  end
+
   it "uses variant price selector class by default" do
     expect(prefs.variant_price_selector_class).to eq Spree::Variant::PriceSelector
   end