Bump postcss and @vue/cli-service in /vue-frontend

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 3.7.0 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
...

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Author: dependabot[bot]