Bump postcss from 8.4.21 to 8.4.31 in /solutions/mint-nft

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](postcss/postcss@8.4.21...8.4.31)

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

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

solutions/mint-nft/package.json

@@ -34,7 +34,7 @@
     "autoprefixer": "^10.4.14",
     "eslint": "^8.36.0",
     "eslint-config-next": "^12.3.4",
-    "postcss": "^8.4.21",
+    "postcss": "^8.4.31",
     "tailwindcss": "^3.2.7",
     "turbo": "^1.8.3",
     "typescript": "4.5.5"