Bump socket.io and maildev in /tests/UI

Bumps [socket.io](https://github.com/socketio/socket.io) and [maildev](https://github.com/maildev/maildev). These dependencies needed to be updated together.

Updates `socket.io` from 4.6.0 to 4.8.1
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/main/CHANGELOG.md)
- [Commits](https://github.com/socketio/socket.io/compare/[email protected])

Updates `maildev` from 2.1.0 to 2.2.1
- [Release notes](https://github.com/maildev/maildev/releases)
- [Commits](maildev/maildev@v2.1.0...v2.2.1)

---
updated-dependencies:
- dependency-name: socket.io
  dependency-type: indirect
- dependency-name: maildev
  dependency-type: indirect
...

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