Add a CI run for the oldest supported Qiskit version

### What should we add?

A CI run testing for the oldest supported Qiskit version may be a good addition for this project. Not having it may cause issue like the one found in #253 where it was caught that `grover_operator` wasn't supported in Qiskit 1.0 before merging, or the more recent #256, where such a bug introduced in #237 went unnoticed.