Bump next from 12.1.0 to 15.0.0 in /examples/nextjs-app

Bumps [next](https://github.com/vercel/next.js) from 12.1.0 to 15.0.0.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v12.1.0...v15.0.0)

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

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