Change Conditional.then_op usage to Conditional.base

[mudit2812]

New work in Pennylane removed Conditional.then_op in favour of Conditional.base. This PR updates lightning to use the new attribute.

[AmintorDusko]

Thank you, @mudit2812!

[multiphaseCFD]

Looks good to me! Thanks @mudit2812 !

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 27.87%. Comparing base (d8baf5a) to head (f990433).
Report is 82 commits behind head on master.

Files with missing lines	Patch %	Lines
...ane_lightning/lightning_kokkos/lightning_kokkos.py	0.00%	1 Missing ⚠️
...nnylane_lightning/lightning_qubit/_state_vector.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (d8baf5a) and HEAD (f990433). Click for more details.

HEAD has 21 uploads less than BASE

Flag	BASE (d8baf5a)	HEAD (f990433)
	25	4

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #752       +/-   ##
===========================================
- Coverage   95.21%   27.87%   -67.35%     
===========================================
  Files         120       15      -105     
  Lines       17504     1729    -15775     
===========================================
- Hits        16667      482    -16185     
- Misses        837     1247      +410     

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