Bump postcss, laravel-mix and resolve-url-loader

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [laravel-mix](https://github.com/JeffreyWay/laravel-mix) and [resolve-url-loader](https://github.com/bholloway/resolve-url-loader/tree/HEAD/packages/resolve-url-loader). 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](postcss/postcss@6.0.23...8.4.31)

Updates `laravel-mix` from 5.0.4 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)

Updates `resolve-url-loader` from 3.1.0 to 3.1.5
- [Release notes](https://github.com/bholloway/resolve-url-loader/releases)
- [Changelog](https://github.com/bholloway/resolve-url-loader/blob/v5/packages/resolve-url-loader/CHANGELOG.md)
- [Commits](https://github.com/bholloway/resolve-url-loader/commits/3.1.5/packages/resolve-url-loader)

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

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