Bump next from 15.5.18 to 15.5.21

Bumps [next](https://github.com/vercel/next.js) from 15.5.18 to 15.5.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](vercel/next.js@v15.5.18...v15.5.21)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.21
  dependency-type: direct:production
...

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

Author: dependabot[bot]

extras/docs/package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@repo/ui": "workspace:^",
-    "next": "^15.5.16",
+    "next": "^15.5.21",
     "react": "^19.2.3",
     "react-dom": "^19.2.3"
   },

extras/web/package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@repo/ui": "workspace:^",
-    "next": "^15.5.16",
+    "next": "^15.5.21",
     "react": "^19.2.3",
     "react-dom": "^19.2.3"
   },