chore(deps): bump next from 15.5.18 to 15.5.21 in /sites/promo (#453)

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](https://github.com/vercel/next.js/compare/v15.5.18...v15.5.21)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

sites/promo/package.json

@@ -17,7 +17,7 @@
     "dayjs": "^1.11.13",
     "feed": "^5.1.0",
     "framer-motion": "^12.23.11",
-    "next": "15.5.18",
+    "next": "15.5.21",
     "react": "^19",
     "react-dom": "^19",
     "react-use-measure": "^2.1.7",