Update node express version - dependabot security (#334)

*Issue description:*
Dependabot security telling us to upgrade express version for node, but
it is unable to generate the PR.

https://github.com/aws-observability/aws-application-signals-test-framework/security/dependabot/17

*Description of changes:*
Updated express to 4.21.2, matching PR as this one:
https://github.com/aws-observability/aws-application-signals-test-framework/pull/330/files

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: harrryr

sample-apps/node/frontend-service/package-lock.json

@@ -13,7 +13,7 @@
     "@aws-sdk/client-s3": "^3.621.0",
     "@types/express": "^4.17.21",
     "@types/node": "^20.14.6",
-    "express": "^4.21.1",
+    "express": "^4.21.2",
     "mysql2": "^3.11.0"
   }
 }