BopesSampler uses deprecated method of VQE - needs to be updated

[woodsp-ibm]

VQE was changed to deprecate a bunch of methods, that were around aspects of the result, to make it stateless, which now requires the result object to be used directly. AdaptVQE was updated in #222 but as that was progressed, after the Terra PR to make VQE stateless was merged, CI failed when BopeSampler accessed one of the deprecated properties. Turned out the decorators were in the wrong order leading it to return different than was expected - see #222 for more details in the comments. That was fixed an CI passed for #222, which has now been merged asap since it was blocking all other activity, but this leaves BopesSampler still needing to be updated to avoid the newly added deprecation of aspects of VQE. The failure was in the BopesSampler tutorial - I recall unit tests passing - indeed in the CI of #222, after it passed, I see no deprecation message in the rollup - so maybe the unit tests don't cover that code (arguably they should) - either way this need investigation and correction. Here is what was saved from the when it showed the message

qiskit_nature/algorithms/pes_samplers/bopes_sampler.py:196: DeprecationWarning: 
The VQE.optimal_params property is deprecated as of Qiskit Terra 0.18.0
and will be removed no sooner than 3 months after the releasedate.
This information is part of the returned result object and can be
queried as VQEResult.optimal_point.
  optimal_params = self._gss.solver.optimal_params  # type: ignore

so its part of this code https://github.com/Qiskit/qiskit-nature/blob/3757845c601a65bc0dad7a6a4eae78ef7a235e2c/qiskit_nature/algorithms/pes_samplers/bopes_sampler.py#L193-L197