Bump postcss and @symfony/webpack-encore

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [@symfony/webpack-encore](https://github.com/symfony/webpack-encore). These dependencies need to be updated together.


Updates `postcss` from 7.0.36 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@7.0.36...8.4.31)

Updates `@symfony/webpack-encore` from 0.28.3 to 4.4.0
- [Release notes](https://github.com/symfony/webpack-encore/releases)
- [Changelog](https://github.com/symfony/webpack-encore/blob/main/CHANGELOG.md)
- [Commits](symfony/webpack-encore@v0.28.3...v4.4.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@symfony/webpack-encore"
  dependency-type: direct:development
...

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