Bump crossbeam-channel from 0.5.4 to 0.5.6

Bumps [crossbeam-channel](https://github.com/crossbeam-rs/crossbeam) from 0.5.4 to 0.5.6.
- [Release notes](https://github.com/crossbeam-rs/crossbeam/releases)
- [Changelog](https://github.com/crossbeam-rs/crossbeam/blob/master/CHANGELOG.md)
- [Commits](crossbeam-rs/crossbeam@crossbeam-channel-0.5.4...crossbeam-channel-0.5.6)

---
updated-dependencies:
- dependency-name: crossbeam-channel
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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