Bump postcss and gulp-autoprefixer

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


Updates `postcss` from 5.2.18 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/commits/8.4.31)

Updates `gulp-autoprefixer` from 3.1.1 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-autoprefixer/releases)
- [Commits](sindresorhus/gulp-autoprefixer@v3.1.1...v8.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gulp-autoprefixer
  dependency-type: direct:production
...

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