Bump semver from 1.0.20 to 1.0.21

Bumps [semver](https://github.com/dtolnay/semver) from 1.0.20 to 1.0.21.
- [Release notes](https://github.com/dtolnay/semver/releases)
- [Commits](dtolnay/semver@1.0.20...1.0.21)

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

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