Bump postcss and next in /nextjs-demo

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [next](https://github.com/vercel/next.js). These dependencies need to be updated together.


Updates `postcss` from 8.4.14 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@8.4.14...8.4.31)

Updates `next` from 13.1.1 to 13.5.6
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v13.1.1...v13.5.6)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

nextjs-demo/package-lock.json

@@ -15,7 +15,7 @@
 		"@types/react-dom": "18.0.10",
 		"eslint": "8.31.0",
 		"eslint-config-next": "13.1.1",
-		"next": "13.1.1",
+		"next": "13.5.6",
 		"react": "18.2.0",
 		"react-dom": "18.2.0",
 		"react-spaces": "file:../dist",