Bump cssnano from 5.1.15 to 6.0.3

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.1.15 to 6.0.3.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.1.15...cssnano@6.0.3)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Author: dependabot[bot]