chore: bump engine.io to version 6.4.0

Diff: socketio/engine.io@6.3.1...6.4.0
Release notes: https://github.com/socketio/engine.io/releases/tag/6.4.0

Author: darrachequesne

package-lock.json

@@ -49,7 +49,7 @@
     "accepts": "~1.3.4",
     "base64id": "~2.0.0",
     "debug": "~4.3.2",
-    "engine.io": "~6.3.1",
+    "engine.io": "~6.4.0",
     "socket.io-adapter": "~2.5.2",
     "socket.io-parser": "~4.2.1"
   },