Update dependency express to ^4.21.1

Author: mend-for-github-com[bot]

package.json

@@ -17,7 +17,7 @@
   "dependencies": {
     "@vonage/server-sdk": "^2.11.2",
     "dotenv": "^16.1.4",
-    "express": "^4.18.2",
+    "express": "^4.21.1",
     "firebase-admin": "^11.9.0",
     "firebase-functions": "^4.4.1",
     "firebase-tools": "^12.3.1",