feat: Remove support for TensorFlow

[matthewfeickert]

Description

• Remove support for TensorFlow from the codebase.
  • This is a breaking change.
  • Remove the TensorFlow backend and optimize/opt_tflow.py.
• Remove use of TensorFlow from the tests.
  • Remove TensorFlow lower bounds from tests/constraints.txt.
• Remove all docs and examples that mention or use TensorFlow.
• Remove mentions of TensorFlow in all parts of the website and REAMDE except for historical purposes like the release notes, JOSS paper, and notes about early pyhf research in FAQs.

Factorizes out parts of PR #2566 and removes additional documentation pieces.

Affected issues

• Resolves Minuit and Tensorflow give different results #876
• Resolves Investigate using TensorFlow v2.0 with lazy evaluation #727
• Resolves Should support for TensorFlow be dropped given its library anti-patterns? #1595

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

* Remove support for TensorFlow from the codebase.
   - This is a breaking change.
   - Remove the TensorFlow backend and optimize/opt_tflow.py.
* Remove use of TensorFlow from the tests.
   - Remove TensorFlow lower bounds from tests/constraints.txt.
* Remove all docs and examples that mention or use TensorFlow.
* Remove mentions of TensorFlow in all parts of the website and REAMDE
  except for historical purposes like the release notes, JOSS paper, and
  notes about early pyhf research in FAQs.

Co-authored-by: Giordon Stark <kratsg@gmail.com>

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (8d23b82) to head (8c55665).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2609      +/-   ##
==========================================
+ Coverage   98.23%   98.25%   +0.01%     
==========================================
  Files          69       67       -2     
  Lines        4545     4360     -185     
  Branches      612      600      -12     
==========================================
- Hits         4465     4284     -181     
+ Misses         48       46       -2     
+ Partials       32       30       -2     

Flag	Coverage Δ
contrib	97.88% <ø> (+0.06%)	⬆️
doctest	98.11% <ø> (+0.01%)	⬆️
unittests-3.10	96.26% <ø> (+<0.01%)	⬆️
unittests-3.11	96.26% <ø> (+<0.01%)	⬆️
unittests-3.12	96.26% <ø> (+<0.01%)	⬆️
unittests-3.8	96.26% <ø> (+<0.01%)	⬆️
unittests-3.9	96.30% <ø> (+<0.01%)	⬆️

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