Bump crossbeam-utils in /2021/plaidctf-2021/proxima-concursus/solver

Bumps [crossbeam-utils](https://github.com/crossbeam-rs/crossbeam) from 0.8.3 to 0.8.8.
- [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-utils-0.8.3...crossbeam-utils-0.8.8)

---
updated-dependencies:
- dependency-name: crossbeam-utils
  dependency-type: indirect
...

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