chore(deps): Bump postcss and @nextcloud/webpack-vue-config

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.6 and updates ancestor dependency [@nextcloud/webpack-vue-config](https://github.com/nextcloud-libraries/webpack-vue-config). 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 `@nextcloud/webpack-vue-config` from 5.4.0 to 7.0.2
- [Release notes](https://github.com/nextcloud-libraries/webpack-vue-config/releases)
- [Changelog](https://github.com/nextcloud-libraries/webpack-vue-config/blob/main/CHANGELOG.md)
- [Commits](nextcloud-libraries/webpack-vue-config@v5.4.0...v7.0.2)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: indirect
- dependency-name: "@nextcloud/webpack-vue-config"
  dependency-version: 7.0.2
  dependency-type: direct:development
...

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Author: dependabot[bot]