chore: bump engine.io-client to version 6.4.0

The minor bump is due to changes on the server side, but we'll keep the
versions in sync.

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

Author: darrachequesne

package-lock.json

@@ -34,7 +34,7 @@
   "dependencies": {
     "@socket.io/component-emitter": "~3.1.0",
     "debug": "~4.3.2",
-    "engine.io-client": "~6.3.1",
+    "engine.io-client": "~6.4.0",
     "socket.io-parser": "~4.2.1"
   },
   "devDependencies": {