Bump mio from 0.8.8 to 0.8.11 in /examples/signaling-server-rust

Bumps [mio](https://github.com/tokio-rs/mio) from 0.8.8 to 0.8.11.
- [Release notes](https://github.com/tokio-rs/mio/releases)
- [Changelog](https://github.com/tokio-rs/mio/blob/master/CHANGELOG.md)
- [Commits](tokio-rs/mio@v0.8.8...v0.8.11)

---
updated-dependencies:
- dependency-name: mio
  dependency-type: indirect
...

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Author: dependabot[bot]