chore: drop Python 3.11 support

[feanil]

Summary

• Drop Python 3.11 support: remove from CI test matrix, tox envlist, and package classifiers
• Regenerate pinned requirements using Python 3.12
• Bump version to 7.0.0 — dropping Python support is a breaking change, so this is a major version bump

Context

Python 3.11 is being dropped across the Open edX ecosystem as part of the move
to standardize on Python 3.12. See the tracking issue for the full list of repos:
openedx/public-engineering#499

Test plan

• CI passes with Python 3.12 only

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.26%. Comparing base (eef8b0f) to head (9bb6b9e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2372   +/-   ##
=======================================
  Coverage   95.26%   95.26%           
=======================================
  Files         196      196           
  Lines       21679    21679           
  Branches     1509     1509           
=======================================
  Hits        20653    20653           
  Misses        776      776           
  Partials      250      250           

Flag	Coverage Δ
unittests	95.26% <100.00%> (ø)

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