Move Order #state_changed method out into Checkout concern (alongside order states logic)

app/models/spree/order.rb

@@ -426,19 +426,6 @@ module Spree
       restart_checkout_flow if state.in?(["payment", "confirmation"])
     end
 
-    def state_changed(name)
-      state = "#{name}_state"
-      return unless persisted?
-
-      old_state = __send__("#{state}_was")
-      state_changes.create(
-        previous_state: old_state,
-        next_state: __send__(state),
-        name: name,
-        user_id: user_id
-      )
-    end
-
     def shipped?
       %w(partial shipped).include?(shipment_state)
     end

app/models/spree/order/checkout.rb

@@ -136,6 +136,19 @@ module Spree
             )
           end
 
+          def state_changed(name)
+            state = "#{name}_state"
+            return unless persisted?
+
+            old_state = __send__("#{state}_was")
+            state_changes.create(
+              previous_state: old_state,
+              next_state: __send__(state),
+              name: name,
+              user_id: user_id
+            )
+          end
+
           private
 
           def validate_payment_method!