Bump nth-check and optimize-css-assets-webpack-plugin (nortonandrews#21)

Removes [nth-check](https://github.com/fb55/nth-check). It's no longer used after updating ancestor dependency [optimize-css-assets-webpack-plugin](https://github.com/NMFR/optimize-css-assets-webpack-plugin). These dependencies need to be updated together.


Removes `nth-check`

Updates `optimize-css-assets-webpack-plugin` from 5.0.3 to 6.0.1
- [Release notes](https://github.com/NMFR/optimize-css-assets-webpack-plugin/releases)
- [Commits](NMFR/optimize-css-assets-webpack-plugin@v5.0.3...v6.0.1)

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: optimize-css-assets-webpack-plugin
  dependency-type: direct:development
...

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