Bump postcss and next

Bumps [postcss](https://github.com/postcss/postcss) and [next](https://github.com/vercel/next.js). These dependencies needed to be updated together.

Updates `postcss` from 8.5.18 to 8.5.23
- [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/8.5.18...8.5.23)

Updates `next` from 15.5.21 to 16.3.0
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](https://github.com/vercel/next.js/compare/v15.5.21...v16.3.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.23
  dependency-type: direct:development
- dependency-name: next
  dependency-version: 16.3.0
  dependency-type: direct:production
...

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

package.json

@@ -19,7 +19,7 @@
     "embla-carousel-react": "^8.1.5",
     "gh-pages": "^6.1.1",
     "lucide-react": "^0.395.0",
-    "next": "15.5.21",
+    "next": "16.3.0",
     "react": "^18",
     "react-dom": "^18",
     "tailwind-merge": "^2.3.0",