Bump postcss from 8.4.21 to 8.4.31 in /frontend

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.21 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/8.4.21...8.4.31)

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

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

frontend/package.json

@@ -48,7 +48,7 @@
     "eslint-plugin-unused-imports": "^2.0.0",
     "husky": "^8.0.0",
     "lint-staged": "^13.1.1",
-    "postcss": "^8.4.21",
+    "postcss": "^8.4.31",
     "prettier": "^2.8.4",
     "prettier-plugin-tailwindcss": "^0.2.2",
     "tailwindcss": "^3.2.4",