Bump postcss and vue-loader in /app

Removes [postcss](https://github.com/postcss/postcss). It's no longer used after updating ancestor dependency [vue-loader](https://github.com/vuejs/vue-loader). These dependencies need to be updated together.


Removes `postcss`

Updates `vue-loader` from 15.11.1 to 17.4.2
- [Release notes](https://github.com/vuejs/vue-loader/releases)
- [Changelog](https://github.com/vuejs/vue-loader/blob/main/CHANGELOG.md)
- [Commits](vuejs/vue-loader@v15.11.1...v17.4.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
  dependency-group: npm_and_yarn-security-group
- dependency-name: vue-loader
  dependency-type: direct:development
  dependency-group: npm_and_yarn-security-group
...

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