Bump next from 15.1.12 to 15.5.21

Bumps [next](https://github.com/vercel/next.js) from 15.1.12 to 15.5.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](vercel/next.js@v15.1.12...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]

package.json

@@ -36,7 +36,7 @@
     "classnames": "^2.2.6",
     "debounce": "^1.2.1",
     "github-slugger": "^1.3.0",
-    "next": "15.1.12",
+    "next": "15.5.21",
     "next-remote-watch": "^1.0.0",
     "parse-numeric-range": "^1.2.0",
     "raw-loader": "^4.0.2",