Bump postcss from 8.4.28 to 8.4.31 in /test/frameworks/next-pnpm

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.28 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@8.4.28...8.4.31)

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

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

test/frameworks/next-pnpm/package.json

@@ -16,7 +16,7 @@
     "eslint": "^8.47.0",
     "eslint-config-next": "^13.4.19",
     "next": "^13.4.19",
-    "postcss": "^8.4.28",
+    "postcss": "^8.4.31",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "tailwindcss": "^3.3.3",