magento/graphql-ce#890: [Checkout] Replace usage of CartItemQuantity with CartItemInterface

Author: lenaorobei

app/code/Magento/QuoteGraphQl/etc/schema.graphqls

@@ -228,9 +228,9 @@ type BillingCartAddress implements CartAddressInterface {
     customer_notes: String @deprecated (reason: "The field is used only in shipping address")
 }
 
-type CartItemQuantity @deprecated(reason: "All fields in CartItemQuantity should be deprecated (so this type can be completely removed in the future releases)") {
-    cart_item_id: Int!
-    quantity: Float!
+type CartItemQuantity @doc(description:"Deprecated: `cart_items` field of `ShippingCartAddress` returns now  `CartItemInterface` instead of `CartItemQuantity`") {
+    cart_item_id: Int! @deprecated(reason: "`cart_items` field of `ShippingCartAddress` returns now `CartItemInterface` instead of `CartItemQuantity`")
+    quantity: Float! @deprecated(reason: "`cart_items` field of `ShippingCartAddress` returns now `CartItemInterface` instead of `CartItemQuantity`")
 }
 
 type CartAddressRegion {