Bump nth-check and cssnano

Bumps [nth-check](https://github.com/fb55/nth-check) to 2.1.1 and updates ancestor dependency [cssnano](https://github.com/cssnano/cssnano). These dependencies need to be updated together.


Updates `nth-check` from 1.0.2 to 2.1.1
- [Release notes](https://github.com/fb55/nth-check/releases)
- [Commits](fb55/nth-check@v1.0.2...v2.1.1)

Updates `cssnano` from 4.1.11 to 6.0.0
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/[email protected])

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: cssnano
  dependency-type: direct:production
...

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