fix: upgrade express from 4.13.4 to 4.21.2

Snyk has created this PR to upgrade express from 4.13.4 to 4.21.2.

See this package in npm:
express

See this project in Snyk:
https://app.snyk.io/org/sammytezzy/project/e3887a0c-2da6-42c4-b2e4-c04b6927f331?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

examples/cluster-traefik/server/package.json

@@ -7,7 +7,7 @@
   "private": true,
   "license": "MIT",
   "dependencies": {
-    "express": "4.13.4",
+    "express": "4.21.2",
     "socket.io": "^4.0.0",
     "socket.io-redis": "^6.0.1"
   },