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 85.79%. Comparing base (984cf32) to head (6a20a0a).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2561   +/-   ##
=======================================
  Coverage   85.79%   85.79%           
=======================================
  Files         247      247           
  Lines       16434    16434           
  Branches     1629     1629           
=======================================
  Hits        14100    14100           
  Misses       1999     1999           
  Partials      335      335           

Flag	Coverage Δ
unittests	85.79% <ø> (ø)

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[pwnage101]

Almost there, looks to be just missing a CHANGELOG.rst entry

[pwnage101]

Approved to unblock