Bump crossbeam-channel from 0.5.11 to 0.5.15 in /frameworks/Rust/rocket

Bumps [crossbeam-channel](https://github.com/crossbeam-rs/crossbeam) from 0.5.11 to 0.5.15.
- [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-0.5.11...crossbeam-channel-0.5.15)

---
updated-dependencies:
- dependency-name: crossbeam-channel
  dependency-version: 0.5.15
  dependency-type: indirect
...

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

Author: dependabot[bot]