Bump postcss, autoprefixer and vue-loader in /client

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [autoprefixer](https://github.com/postcss/autoprefixer) and [vue-loader](https://github.com/vuejs/vue-loader). These dependencies need to be updated together.


Updates `postcss` from 7.0.17 to 8.4.21
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.17...8.4.21)

Updates `autoprefixer` from 6.7.7 to 10.4.14
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@6.7.7...10.4.14)

Updates `vue-loader` from 11.3.4 to 17.0.1
- [Release notes](https://github.com/vuejs/vue-loader/releases)
- [Changelog](https://github.com/vuejs/vue-loader/blob/next/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-loader/commits/v17.0.1)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: autoprefixer
  dependency-type: direct:development
- dependency-name: vue-loader
  dependency-type: direct:development
...

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Author: dependabot[bot]