Operator Arithmetic support to qubit_observable() in qchem

[Jaybsoni]

Context:
Now that the new operator arithmetic machinery is integrated with pennylane (via enable_new_opmath()), we want to start making use of it across the codebase, specifically in the qchem module.

Description of the Change:
Update the qubit_observable() function in observable_hf.py to use operator arithmetic when active_new_opmath() is True.

Benefits:
We are slowly moving away from Hamiltonian and adopting the new machinery we have!

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #4138 (49aa27c) into master (09f0af3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4138   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files         347      347           
  Lines       31416    31442   +26     
=======================================
+ Hits        31345    31371   +26     
  Misses         71       71           

Impacted Files	Coverage Δ
pennylane/ops/qubit/attributes.py	100.00% <ø> (ø)
pennylane/__init__.py	100.00% <100.00%> (ø)
pennylane/fermi/__init__.py	100.00% <100.00%> (ø)
pennylane/fermi/fermionic.py	100.00% <100.00%> (ø)
pennylane/ops/op_math/exp.py	100.00% <100.00%> (ø)
pennylane/qchem/observable_hf.py	98.93% <100.00%> (+0.09%)	⬆️

[Jaybsoni]

[sc-38630]

[lillian542]

Looks good to me! 🚀

Suggested a correction to an outdated example output, and one question :)

[lillian542]

Nice! 🎉