Update dependency socket.io to v4

Author: renovate-bot

plugin/multiplex/package.json

@@ -13,7 +13,7 @@
     "express": "4.13.4",
     "grunt-cli": "0.1.13",
     "mustache": "2.2.1",
-    "socket.io": "1.3.7"
+    "socket.io": "4.0.1"
   },
   "license": "MIT"
 }