Bump postcss and vanilla-framework in /themes/ubuntukr

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


Removes `postcss`

Updates `vanilla-framework` from 3.4.0 to 3.15.1
- [Release notes](https://github.com/canonical/vanilla-framework/releases)
- [Changelog](https://github.com/canonical/vanilla-framework/blob/main/releases.yml)
- [Commits](canonical/vanilla-framework@v3.4.0...v3.15.1)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: vanilla-framework
  dependency-type: direct:production
...

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