build(deps): bump postcss and @vue/cli-service in /demo/vue-cli3

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.20 and updates ancestor dependency [@vue/cli-service](https://github.com/vuejs/vue-cli/tree/HEAD/packages/@vue/cli-service). These dependencies need to be updated together.


Updates `postcss` from 6.0.23 to 8.4.20
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...8.4.20)

Updates `@vue/cli-service` from 3.12.1 to 5.0.8
- [Release notes](https://github.com/vuejs/vue-cli/releases)
- [Changelog](https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-cli/commits/v5.0.8/packages/@vue/cli-service)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@vue/cli-service"
  dependency-type: direct:development
...

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

Author: dependabot[bot]