Drop Python 3.11 and NumPy <2.0 per SPEC-0

[maxrjones]

Apply scientific-python SPEC-0 support policy:

• Python minimum 3.11 -> 3.12 (3.11 released Oct 2022, 3-year window expired)
• NumPy minimum 1.24 -> 2.0 (1.24 released Dec 2022, 2-year window expired)
• Zarr test pins 3.1.2/3.1.3 -> 3.0.8/3.1.6 (oldest supported + latest)

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8b882dc) to head (6b855ac).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #834   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines          902       901    -1     
=========================================
- Hits           902       901    -1     

Files with missing lines	Coverage Δ
src/numcodecs/checksum32.py	100.00% <100.00%> (ø)

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