Bump postcss and laravel-mix

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [laravel-mix](https://github.com/JeffreyWay/laravel-mix). These dependencies need to be updated together.


Updates `postcss` from 6.0.23 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/6.0.23...8.4.31)

Updates `laravel-mix` from 4.0.15 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] <support@github.com>

package.json

@@ -16,7 +16,7 @@
         "cross-env": "^5.1",
         "imagemin": "^6.0.0",
         "jquery": ">=3.4.0",
-        "laravel-mix": "^4.0.15",
+        "laravel-mix": "^6.0.49",
         "lodash": "^4.17.5",
         "popper.js": "^1.14.7",
         "resolve-url-loader": "^2.3.2",