build(deps-dev): bump postcss from 8.4.14 to 8.4.31 in /demos (#4508)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.14 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.4.14...8.4.31)

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

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demos/package.json

@@ -30,7 +30,7 @@
     "@vitejs/plugin-vue": "^1.10.2",
     "autoprefixer": "^10.4.2",
     "iframe-resizer": "^4.3.2",
-    "postcss": "^8.4.6",
+    "postcss": "^8.4.31",
     "postcss-import": "^15.1.0",
     "react": "^18.0.0",
     "react-dom": "^18.0.0",