Patch of _reduceStock() under @sap/cds 4 (#136)

* Bump to cds 4
* Cosmetics in handler code
  * await and cds.run([...]) instead of Promise.all

Co-authored-by: Christian Georgi <christian.georgi@sap.com>

package.json

@@ -11,7 +11,7 @@
     "bookshop": "cds watch packages/bookshop"
   },
   "dependencies": {
-    "@sap/cds": "^3",
+    "@sap/cds": "^4",
     "express": "^4"
   },
   "devDependencies": {