Bump snapbox from 0.6.20 to 0.6.21

Bumps [snapbox](https://github.com/assert-rs/snapbox) from 0.6.20 to 0.6.21.
- [Commits](assert-rs/snapbox@snapbox-v0.6.20...snapbox-v0.6.21)

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

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