Migrate to Qiskit Nature v0.5

[towynlin]

When running the notebook code locally, I received this deprecation warning:

NatureDeprecationWarning: The qiskit_nature.operators package is deprecated as of version 0.5.0 and will be removed no sooner than 3 months after the release. Instead use the qiskit_nature.second_q.operators package.

I mentioned this in slack, and Steve Wood helpfully pointed me to the Qiskit Nature v0.5 Migration Guide.

The HeisenbergModel is almost identical to the IsingModel, so I subclassed it and made the minor override in second_q_op. The notebook then just needed a couple imports changed and the usage of HeisenbergModel simplified.

I don't have access to ibmq_quadalupe or the included provider, so I can't run the whole notebook — just running simulations for now. Someone who does have access should probably check that everything runs well. In an effort to maintain compatibility and clear messaging, I minimized what I changed in the notebook — just enough to make sure the code using the updated imports ran as expected.

[CLAassistant]

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