chore(deps): bump postcss and vue-loader in /packages/webpack5 (NativeScript#10772)

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.6 and updates ancestor dependency [vue-loader](https://github.com/vuejs/vue-loader). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.5.6
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.5.6)

Updates `vue-loader` from 15.9.8 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@v15.9.8...v17.4.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: indirect
- dependency-name: vue-loader
  dependency-version: 17.4.2
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]