Update code under `demos`

### Environment

- **Qiskit version**:
- **Python version**:
- **Operating system**:


### What is happening?

The demos code uses updated dependencies (qiskit-optimization) and classes (QAOAAnsatz). It should be updated for consistency with the rest of the repo.

### How can we reproduce the issue?

 Seen during QDC.

### What should happen?

Updated code.

### Any suggestions?

  