Docs: Migration guide should use Qiskit Runtime

[jyu00]

The migration guide uses from qiskit.primitives import Sampler, which uses a local statevector simulator. But people migrating their Qiskit Runtime custom programs would not want to use a local simulator. They'd want to use a real IBM Quantum backend (otherwise they can just run locally on their laptop).

[jyu00]

cc @jenglick

[Tansito]

Thanks for the ping @jyu00 🙏

We will take a look