Update dependency socket.io to v4

Author: renovate-bot

plugin/multiplex/package.json

@@ -13,7 +13,7 @@
     "express": "~4.13.3",
     "grunt-cli": "~0.1.13",
     "mustache": "~2.2.1",
-    "socket.io": "~1.3.7"
+    "socket.io": "4.3.0"
   },
   "license": "MIT"
 }