Bump tokio from 0.1.20 to 1.8.4 in /workshop_serveless/ex4

Bumps [tokio](https://github.com/tokio-rs/tokio) from 0.1.20 to 1.8.4.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](tokio-rs/tokio@tokio-0.1.20...tokio-1.8.4)

---
updated-dependencies:
- dependency-name: tokio
  dependency-type: direct:production
...

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