Update to workbox-webpack-plugin v4 (#6725)

* Update to workbox-webpack-plugin v4

* Update workbox-webpack-plugin to 4.2.0


Co-authored-by: Ian Sutherland <ian@iansutherland.ca>

Author: r0ughnex

packages/react-scripts/package.json

@@ -71,7 +71,7 @@
     "webpack": "4.29.6",
     "webpack-dev-server": "3.2.1",
     "webpack-manifest-plugin": "2.0.4",
-    "workbox-webpack-plugin": "3.6.3"
+    "workbox-webpack-plugin": "4.2.0"
   },
   "devDependencies": {
     "react": "^16.8.4",