Bump rodio from 0.20.1 to 0.21.0

Bumps [rodio](https://github.com/RustAudio/rodio) from 0.20.1 to 0.21.0.
- [Changelog](https://github.com/RustAudio/rodio/blob/master/CHANGELOG.md)
- [Commits](RustAudio/rodio@v0.20.1...v0.21.0)

---
updated-dependencies:
- dependency-name: rodio
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]