chore(deps-dev): bump @turbo/gen in /templates/next-monorepo

Bumps [@turbo/gen](https://github.com/vercel/turborepo/tree/HEAD/packages/turbo-gen) from 2.8.7 to 2.8.16.
- [Release notes](https://github.com/vercel/turborepo/releases)
- [Changelog](https://github.com/vercel/turborepo/blob/main/RELEASE.md)
- [Commits](https://github.com/vercel/turborepo/commits/v2.8.16/packages/turbo-gen)

---
updated-dependencies:
- dependency-name: "@turbo/gen"
  dependency-version: 2.8.16
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

templates/next-monorepo/packages/ui/package.json

@@ -16,7 +16,7 @@
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4.1.18",
-    "@turbo/gen": "^2.8.1",
+    "@turbo/gen": "^2.8.16",
     "@types/node": "^25.1.0",
     "@types/react": "^19.2.10",
     "@types/react-dom": "^19.2.3",