Bump postcss, @nuxt/types and nuxt

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [@nuxt/types](https://github.com/nuxt/nuxt.js) and [nuxt](https://github.com/nuxt/nuxt). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.31)

Updates `@nuxt/types` from 2.15.8 to 2.17.1
- [Release notes](https://github.com/nuxt/nuxt.js/releases)
- [Commits](nuxt/nuxt@v2.15.8...v2.17.1)

Updates `nuxt` from 2.15.8 to 2.17.1
- [Release notes](https://github.com/nuxt/nuxt/releases)
- [Commits](nuxt/nuxt@v2.15.8...v2.17.1)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@nuxt/types"
  dependency-type: indirect
- dependency-name: nuxt
  dependency-type: direct:production
...

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