Bump futures-util in /code-samples/serving/cloudevents/cloudevents-rust

Bumps [futures-util](https://github.com/rust-lang/futures-rs) from 0.3.5 to 0.3.28.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.5...0.3.28)

---
updated-dependencies:
- dependency-name: futures-util
  dependency-type: indirect
...

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