Bump next from 14.2.7 to 14.2.21 in /docs

Bumps [next](https://github.com/vercel/next.js) from 14.2.7 to 14.2.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](https://github.com/vercel/next.js/compare/v14.2.7...v14.2.21)

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

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

docs/package.json

@@ -5,7 +5,7 @@
     "start": "NEXT_TELEMETRY_DISABLED=1 next start"
   },
   "dependencies": {
-    "next": "^14.2.7",
+    "next": "^14.2.21",
     "nextra": "^2.13.4",
     "nextra-theme-docs": "^2.13.4",
     "react": "^18.3.1",