Bump postcss from 7.0.35 to 7.0.39 in /webcomponents/ui.frontend.vanilla

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.35 to 7.0.39.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/7.0.39/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.35...7.0.39)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

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