Use rpcauth instead of rpc-user and rpc-password

I propose addressing the warning:
```bash
2025-07-13T11:23:43Z WARNING: the RPC server is not safe to expose to untrusted networks such as the public internet
2025-07-13T11:23:43Z Config options rpcuser and rpcpassword will soon be deprecated. Locally-run instances may remove rpcuser to use cookie-based auth, or may be replaced with rpcauth.
```

