build(deps): bump clap_mangen from 0.2.20 to 0.2.21 (#293)

Bumps [clap_mangen](https://github.com/clap-rs/clap) from 0.2.20 to 0.2.21.
- [Release notes](https://github.com/clap-rs/clap/releases)
- [Changelog](https://github.com/clap-rs/clap/blob/master/CHANGELOG.md)
- [Commits](clap-rs/clap@clap_mangen-v0.2.20...clap_mangen-v0.2.21)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Pavel Ivanov <mr.pavel.ivanov@gmail.com>