chore(deps): bump postcss and vue-loader in /build/frontend-legacy

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.23 and updates ancestor dependency [vue-loader](https://github.com/vuejs/vue-loader). These dependencies need to be updated together.


Updates `postcss` from 8.5.16 to 8.5.23
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.16...8.5.23)

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-version: 8.5.23
  dependency-type: indirect
- dependency-name: vue-loader
  dependency-version: 17.4.2
  dependency-type: direct:development
...

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

Author: dependabot[bot]