Bump socket.io from 2.3.0 to 2.4.0 in /back-end

Bumps [socket.io](https://github.com/socketio/socket.io) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/2.4.0/CHANGELOG.md)
- [Commits](socketio/socket.io@2.3.0...2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

back-end/package-lock.json

@@ -34,7 +34,7 @@
     "passport-google-oauth20": "^2.0.0",
     "rebuild": "^0.1.2",
     "request": "^2.88.2",
-    "socket.io": "^2.3.0",
+    "socket.io": "^2.4.0",
     "socketio-auth": "^0.1.1",
     "youtube-search": "^1.1.4"
   },