Bump cssnano from 5.0.5 to 5.0.6

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.0.5 to 5.0.6.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.0.5...cssnano@5.0.6)

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

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Author: dependabot[bot]