Upgrade to Qiskit Nature 0.6 features once it releases

[mrossinek]

What should we add?

Qiskit Nature 0.6 is loosely planned to be released mid of April this year. Once that happens, we should update the examples and unittests here accordingly:

• replace usage of QubitConverter directly with QubitMapper instances (see QubitConverter Refactoring qiskit-nature#967)
• replace the usage of VQEUCCFactory (or other eigensolver factories) (see Deprecate the (Minimum)EigensolverFactory classes qiskit-nature#1084)
• make use of the new symmetry-reduced integral container classes (see Adds symmetry-reduced two-body electronic integral utilities qiskit-nature#1072) extracted to Leverage Qiskit Nature's SymmetryTwoBodyIntegrals classes #25

This may well be split into multiple issues but I figured I should collect the improvements available via 0.6 in one place for the time being.

ℹ️ This will be on hold until Qiskit Nature 0.6 is actually released.