CartUpdateItemResource and CartUpdateItemsResource should use \Drupal\commerce_cart\CartManager to update any order items for an order.

Updating order items for a order without calling \Drupal\commerce_cart\CartManager::updateOrderItem will miss some important task in commerce_cart module, for example:

1. CartOrderItemUpdateEvent will not be trigered.
2. CheckoutFlow and CheckoutFlow steps will not be reset.
3. If some custom module overide the @commerce_cart.cart_manager service to provide additional code to CartManager::updateOrderItem, it will not working.