Bump github.com/smallstep/certificates from 0.28.1 to 0.28.2

Bumps [github.com/smallstep/certificates](https://github.com/smallstep/certificates) from 0.28.1 to 0.28.2.
- [Release notes](https://github.com/smallstep/certificates/releases)
- [Changelog](https://github.com/smallstep/certificates/blob/master/CHANGELOG.md)
- [Commits](https://github.com/smallstep/certificates/compare/v0.28.1...v0.28.2

)

---
updated-dependencies:
- dependency-name: github.com/smallstep/certificates
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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