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>

packages/bookshop/package.json

@@ -4,10 +4,10 @@
     "description": "A simple bookshop application, build in a self-contained all-in-one fashion, i.e. w/o reusing other packages.",
     "license": "SAP SAMPLE CODE LICENSE",
     "dependencies": {
-       "@sap/cds": "^3",
-       "express": "^4",
-       "passport": "^0.4.1",
-       "hdb": "^0.17.1"
+        "@sap/cds": "^4",
+        "express": "^4",
+        "hdb": "^0.18.1",
+        "passport": "^0.4.1"
     },
     "scripts": {
         "start": "cds run --in-memory?",