Update casting for apply_operation

[mudit2812]

• Update casting for matrices in qml.devices.qubit.apply_operation. For some reason the old casting was making jax happy but not autograd.
• Remove the 0.38.1 changelog. @josh146 confirmed that we should do this now that the release has been yanked from PyPi.

[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.

[mudit2812]

I'm going to open a temporary PR in qml to promote ComplexWarnings to errors and install this branch to verify that all warnings are fixed after this PR is merged.

[PietropaoloFrisoni]

I'm going to open a temporary PR in qml to promote ComplexWarnings to errors and install this branch to verify that all warnings are fixed after this PR is merged.

Makes sense! Are these failures expected?

[mudit2812]

I'm going to open a temporary PR in qml to promote ComplexWarnings to errors and install this branch to verify that all warnings are fixed after this PR is merged.

Makes sense! Are these failures expected?

@PietropaoloFrisoni In hindsight, I should've expected those failures. I've fixed them now by having a separate branch for casting when the interface is "tensorflow".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (de54c54) to head (6ccb3fb).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6268   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files         444      444           
  Lines       42133    42202   +69     
=======================================
+ Hits        42008    42079   +71     
+ Misses        125      123    -2     

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