Bump postcss in /examples/nextjs-response-streaming (#289)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.20 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@8.4.20...8.4.31)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/nextjs-response-streaming/package.json

@@ -23,7 +23,7 @@
     "eslint-config-next": "13.3.0",
     "ms": "3.0.0-canary.1",
     "next": "13.3.0",
-    "postcss": "8.4.21",
+    "postcss": "8.4.31",
     "react": "18.2.0",
     "react-dom": "18.2.0",
     "sharp": "^0.32.0",
@@ -41,7 +41,7 @@
     "eslint": "8.30.0",
     "eslint-config-next": "13.1.0",
     "lint-staged": "13.1.0",
-    "postcss": "8.4.20",
+    "postcss": "8.4.31",
     "prettier": "2.8.1",
     "prettier-plugin-tailwindcss": "0.2.1",
     "tailwindcss": "3.3.1",