fix: exercises/c4-client-master/package.json & exercises/c4-client-master/package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-BABELRUNTIME-10044504

Author: snyk-bot

exercises/c4-client-master/package-lock.json

@@ -8,7 +8,7 @@
     "immutability-helper": "^3.0.1",
     "react": "^16.8.6",
     "react-dom": "^16.8.6",
-    "react-router-dom": "^5.0.1",
+    "react-router-dom": "^6.4.0",
     "react-scripts": "5.0.0",
     "semantic-ui-css": "^2.4.1",
     "semantic-ui-react": "^0.86.0",