"approximation" argument of Estimator is poorly named

[kevinsung]

What is the expected behavior?

It's somewhat confusing that setting approximation=True is necessary to get the exact expectation value. For example, consider this sentence from the documentation:

If None and approximation is True, it calculates the exact expectation values.

I suggest renaming the argument to simulate_sampling_error or just simulate_error.

[athanzxyt]

Hi @hhorii this PR is ready for review.

[ikkoham]

Yes. This requires some DeprecationWarning. I'll redesign overall with options (with flexible dataclass) align with qiskit-ibm-runtime.