Bump postcss in /example_applications/web_app/assets (#217)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.7 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@8.4.7...8.4.31)

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

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example_applications/web_app/assets/package.json

@@ -30,7 +30,7 @@
     "mini-css-extract-plugin": "^0.9.0",
     "nanoid": "^3.1.31",
     "optimize-css-assets-webpack-plugin": "^5.0.1",
-    "postcss": ">=8.4.7",
+    "postcss": ">=8.4.31",
     "sass": "^1.22.10",
     "sass-loader": "^8.0.2",
     "ssri": ">=6.0.2",