chore(backport): Update services and docs to Python 3.12 as default

[matthewfeickert]

Description

Backport parts of PR #2461

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Backport parts of PR https://github.com/scikit-hep/pyhf/pull/2461

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release/v0.7.x@99c28fb). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2504   +/-   ##
=================================================
  Coverage                  ?   98.19%           
=================================================
  Files                     ?       69           
  Lines                     ?     4547           
  Branches                  ?      804           
=================================================
  Hits                      ?     4465           
  Misses                    ?       49           
  Partials                  ?       33           

Flag	Coverage Δ
contrib	97.73% <ø> (?)
doctest	98.02% <ø> (?)
unittests-3.10	96.19% <ø> (?)
unittests-3.11	96.19% <ø> (?)
unittests-3.12	96.19% <ø> (?)
unittests-3.7	96.17% <ø> (?)
unittests-3.8	96.21% <ø> (?)
unittests-3.9	96.23% <ø> (?)

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