chore(deps-dev): bump tsdown from 0.11.13 to 0.21.1 in /packages/next

Bumps [tsdown](https://github.com/rolldown/tsdown) from 0.11.13 to 0.21.1.
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.11.13...v0.21.1)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.21.1
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

packages/next/package.json

@@ -69,7 +69,7 @@
     "next": "^15.0.0",
     "react": "^19.2.3",
     "react-dom": "^19.2.3",
-    "tsdown": "^0.11.9",
+    "tsdown": "^0.21.1",
     "typescript": "^5.4.0",
     "vitest": "^3.2.4"
   },