examples: update `with-supabase` example to App Router (#51335)
### What?
Update Next.js with Supabase example
### Why?
Existing example for Next.js with Supabase is out of date
### How?
- Rename `with-supabase-auth-db-realtime` to `with-supabase`
- Update example to use App Router
- Use Supabase Auth Helpers for Next.js to configure auth cookies
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