Update webpack-dev-server to 3.2.1 (#6483)

* Upgrade webpack-dev-server to 3.2.1

Author: ThePrez

Diff for: packages/react-scripts/package.json

@@ -67,7 +67,7 @@
     "terser-webpack-plugin": "1.2.2",
     "url-loader": "1.1.2",
     "webpack": "4.28.3",
-    "webpack-dev-server": "3.1.14",
+    "webpack-dev-server": "3.2.1",
     "webpack-manifest-plugin": "2.0.4",
     "workbox-webpack-plugin": "3.6.3"
   },