Bump postcss, css-loader and vue-loader

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


Updates `postcss` from 5.2.18 to 8.4.21
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/8.4.21)

Updates `css-loader` from 0.28.11 to 6.7.3
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/css-loader@v0.28.11...v6.7.3)

Updates `vue-loader` from 13.7.3 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: css-loader
  dependency-type: direct:development
- dependency-name: vue-loader
  dependency-type: direct:development
...

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