build(deps): bump postcss and ember-cli-autoprefixer

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


Updates `postcss` from 8.4.19 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@8.4.19...8.4.31)

Updates `ember-cli-autoprefixer` from 1.0.3 to 2.0.0
- [Release notes](https://github.com/kimroen/ember-cli-autoprefixer/releases)
- [Changelog](https://github.com/kimroen/ember-cli-autoprefixer/blob/master/CHANGELOG.md)
- [Commits](kimroen/ember-cli-autoprefixer@v1.0.3...v2.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: ember-cli-autoprefixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]