chore(deps): bump mio from 0.7.14 to 0.8.11 in /backchannel (#1738)

Bumps [mio](https://github.com/tokio-rs/mio) from 0.7.14 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@v0.7.14...v0.8.11)

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

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