fix: Remove deprecated import path for QubitDevice

[astralcai]

Issue #, if available:

Description of changes:

The QubitDevice class is no longer accessible via the top level of PennyLane (see PennyLaneAI/pennylane#6537)

Testing done:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4105263) to head (13dd66f).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #281   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1250      1251    +1     
  Branches       163       163           
=========================================
+ Hits          1250      1251    +1     

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