build(deps): bump postcss and preact-cli in /example/frontend

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [preact-cli](https://github.com/preactjs/preact-cli/tree/HEAD/packages/cli). 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 `preact-cli` from 2.2.1 to 3.5.0
- [Release notes](https://github.com/preactjs/preact-cli/releases)
- [Changelog](https://github.com/preactjs/preact-cli/blob/master/packages/cli/CHANGELOG.md)
- [Commits](https://github.com/preactjs/preact-cli/commits/[email protected]/packages/cli)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: preact-cli
  dependency-type: direct:production
...

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