build(deps-dev): bump css-minimizer-webpack-plugin in /superset-frontend

Bumps [css-minimizer-webpack-plugin](https://github.com/webpack-contrib/css-minimizer-webpack-plugin) from 3.4.1 to 6.0.0.
- [Release notes](https://github.com/webpack-contrib/css-minimizer-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/css-minimizer-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/css-minimizer-webpack-plugin@v3.4.1...v6.0.0)

---
updated-dependencies:
- dependency-name: css-minimizer-webpack-plugin
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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