Bump engine.io and socket.io in /backend

Bumps [engine.io](https://github.com/socketio/engine.io) to 3.6.1 and updates ancestor dependency [socket.io](https://github.com/socketio/socket.io). These dependencies need to be updated together.


Updates `engine.io` from 3.1.4 to 3.6.1
- [Release notes](https://github.com/socketio/engine.io/releases)
- [Changelog](https://github.com/socketio/engine.io/blob/main/CHANGELOG.md)
- [Commits](socketio/engine.io@3.1.4...3.6.1)

Updates `socket.io` from 2.0.4 to 2.5.0
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/main/CHANGELOG.md)
- [Commits](socketio/socket.io@2.0.4...2.5.0)

---
updated-dependencies:
- dependency-name: engine.io
  dependency-type: indirect
- dependency-name: socket.io
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>