Deprecate max expansion from qml.device and qml.QNode

[EmilianoG-byte]

Context:

max_expansion only serves to prevent decompositions from reach the gateset they need to reach. Therefore, we are deprecating it in 0.38 and removing it in 0.39.

Description of the Change:

The max_expansion is deprecated from qml.QNode and the analogous decomp_depth is deprecated from qml.device

Benefits:

Specifying this only prevents the decomposition for reaching the target gateset, leading to things falling over uncontrollably later. Deprecating it (and later removing it) will avoid this.

Possible Drawbacks:

Users currently utilizing this argument must update their code after 0.39.

[sc-64693]

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (f454f59) to head (2ff6a7e).
Report is 293 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6026      +/-   ##
==========================================
- Coverage   99.66%   99.65%   -0.01%     
==========================================
  Files         427      427              
  Lines       41059    40774     -285     
==========================================
- Hits        40920    40634     -286     
- Misses        139      140       +1     

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[mudit2812]

One blocking comment left above for now. I'll give a 2nd pass after this is addressed.

[PietropaoloFrisoni]

It looks all good to me!