Update cssnano requirement from 4.1.7 to 4.1.8 (#3880)

Updates the requirements on [cssnano](https://github.com/cssnano/cssnano) to permit the latest version.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/commits/v4.1.8

)

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