Bump next from 9.5.5 to 12.1.0

Bumps [next](https://github.com/vercel/next.js) from 9.5.5 to 12.1.0.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v9.5.5...v12.1.0)

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

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

Author: dependabot[bot]

packages/demo/package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "@zeit/next-css": "^1.0.1",
-    "next": "^9.5.4",
+    "next": "^12.1.0",
     "next-plugin-preact": "^3.0.2",
     "preact": "^10.4.6",
     "preact-render-to-string": "^5.1.10",