Require Python >=3.12, drop 3.11

[timtreis]

Mirrors scverse/spatialdata#1151. anndata>=0.12 already requires Python >=3.12, so 3.11 is effectively unsupported.

• requires-python >=3.11 -> >=3.12; drop the 3.11 classifier
• .mypy.ini python_version 3.10 -> 3.12
• hatch test matrix 3.11 -> 3.12 (stays 3.12 + 3.14)
• pixi lanes py311/dev-py311/docs-py311 -> py312/dev-py312/docs-py312
• README install note 3.10 -> 3.12

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.59%. Comparing base (50a6606) to head (cba09a7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #743   +/-   ##
=======================================
  Coverage   79.59%   79.59%           
=======================================
  Files          17       17           
  Lines        4641     4641           
  Branches     1029     1029           
=======================================
  Hits         3694     3694           
  Misses        598      598           
  Partials      349      349           

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