Bump postcss and laravel-mix

Bumps [postcss](https://github.com/postcss/postcss) and [laravel-mix](https://github.com/JeffreyWay/laravel-mix). These dependencies needed to be updated together.

Updates `postcss` from 6.0.23 to 8.4.16
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.16)

Updates `laravel-mix` from 4.1.4 to 6.0.49
- [Release notes](https://github.com/JeffreyWay/laravel-mix/releases)
- [Changelog](https://github.com/laravel-mix/laravel-mix/blob/master/CHANGELOG.md)
- [Upgrade guide](https://github.com/laravel-mix/laravel-mix/blob/master/UPGRADE.md)
- [Commits](https://github.com/JeffreyWay/laravel-mix/commits)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: laravel-mix
  dependency-type: direct:development
...

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