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:

• [x] replace usage of QubitConverter directly with QubitMapper instances (see https://github.com/Qiskit/qiskit-nature/issues/967)
• [x] replace the usage of VQEUCCFactory (or other eigensolver factories) (see https://github.com/Qiskit/qiskit-nature/pull/1084)
• [x] ~make use of the new symmetry-reduced integral container classes (see https://github.com/Qiskit/qiskit-nature/pull/1072)~ extracted to #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.

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