fix(deps): update dependency isomorphic-unfetch to v4

Author: renovate[bot]

package-lock.json

@@ -15,7 +15,7 @@
   },
   "dependencies": {
     "babel-plugin-istanbul": "5.2.0",
-    "isomorphic-unfetch": "3.1.0",
+    "isomorphic-unfetch": "4.0.2",
     "next": "9.5.5",
     "react": "16.14.0",
     "react-dom": "16.14.0"