fix: Add poi_name to Model construction in notebooks

[matthewfeickert]

Description

• Ensure all pyhf.Model calls in the example notebooks that don't already have a poi_name definied have poi_name="mu" set.
• Amends PR feat: Don't specify a default POI name #2328

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Ensure all pyhf.Model calls in the example notebooks that don't
  already have a poi_name definied have `poi_name="mu"` set.
* Amends PR https://github.com/scikit-hep/pyhf/pull/2328

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Codecov Report

Patch and project coverage have no change.

Comparison is base (4ec7bf3) 98.28% compared to head (81f2756) 98.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2336   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
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  Files          69       69           
  Lines        4538     4538           
  Branches      803      803           
=======================================
  Hits         4460     4460           
  Misses         45       45           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.86% <ø> (ø)
doctest	60.99% <ø> (ø)
unittests-3.10	96.29% <ø> (ø)
unittests-3.11	96.29% <ø> (ø)
unittests-3.8	96.31% <ø> (ø)
unittests-3.9	96.34% <ø> (ø)

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[matthewfeickert]

The build failure was addressed in PR #2337, so this is fine to merge.