build(deps-dev): bump postcss from 7.0.17 to 8.4.31 in /assets/uikit

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.17 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.17...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.4.31
  dependency-type: direct:development
...

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

Author: dependabot[bot]

assets/uikit/package.json

@@ -48,7 +48,7 @@
     "less": "^2.7.2",
     "minimist": "^1.2.0",
     "npm-watch": "github:dasdeck/npm-watch#46a8a36c61cfd998ed53d27f62eaf79e11c395b4",
-    "postcss": "7.0.17",
+    "postcss": "8.4.31",
     "raw-loader": "^3.0.0",
     "rollup": "1.15.6",
     "rollup-plugin-buble": "^0.19.2",