fix: Remove use of MeasurementProcess.return_type

[albi3ro]

Issue #, if available:

See Issue #289

Description of changes:

MeasurementProcess.return_type and the associated ObservableReturnType enum was deprecated in pennylane v0.41 and has now been removed in latest PennyLane. This PR updates the plugin to use the recommended isinstance checks instead of checking the removed return_type property.

Testing done:

I ran all but the test_ops.py unit tests locally. The required version of tensorflow was for test_ops.py was not available for my mac.

But other than that, the test seem fine with pennylane master installed.

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b578a4c) to head (94fa03d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #288   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1244      1245    +1     
  Branches       161       162    +1     
=========================================
+ Hits          1244      1245    +1     

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