fix(backport): Replace deprecated np.product by np.prod

[matthewfeickert]

Description

• Backport PR fix: Replace deprecated np.product by np.prod #2242
• Use np.prod API over np.product as np.product is deprecated as of NumPy v1.25.0.
  • c.f. https://numpy.org/devdocs/release/1.25.0-notes.html#deprecations

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• Tests are passing
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* Backport PR https://github.com/scikit-hep/pyhf/pull/2242
* Use np.prod API over np.product as np.product is deprecated
  as of NumPy v1.25.0.
   - c.f. https://numpy.org/devdocs/release/1.25.0-notes.html#deprecations

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Codecov Report

❗ No coverage uploaded for pull request base (release/v0.7.x@3639975). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2284   +/-   ##
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  Coverage                  ?   98.23%           
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  Files                     ?       69           
  Lines                     ?     4538           
  Branches                  ?      801           
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  Hits                      ?     4458           
  Misses                    ?       47           
  Partials                  ?       33           

Flag	Coverage Δ
contrib	27.54% <0.00%> (?)
doctest	61.06% <0.00%> (?)
unittests-3.10	96.27% <0.00%> (?)
unittests-3.11	96.27% <0.00%> (?)
unittests-3.7	96.25% <0.00%> (?)
unittests-3.8	96.29% <0.00%> (?)
unittests-3.9	96.31% <0.00%> (?)

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