chore: remove zarr v2 the package + default to v3 format

[ilan-gold]

• Closes #
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

❌ Patch coverage is 96.36364% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.55%. Comparing base (2a2c0e3) to head (f9bbce5).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/_core/sparse_dataset.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2309      +/-   ##
==========================================
- Coverage   86.68%   84.55%   -2.13%     
==========================================
  Files          46       46              
  Lines        7203     7150      -53     
==========================================
- Hits         6244     6046     -198     
- Misses        959     1104     +145     

Files with missing lines	Coverage Δ
src/anndata/_io/specs/methods.py	90.99% <100.00%> (+0.06%)	⬆️
src/anndata/_io/specs/registry.py	94.55% <100.00%> (ø)
src/anndata/_io/zarr.py	79.76% <100.00%> (-3.04%)	⬇️
src/anndata/_settings.py	92.77% <100.00%> (+0.86%)	⬆️
src/anndata/compat/__init__.py	84.90% <100.00%> (-4.55%)	⬇️
src/anndata/experimental/backed/_io.py	96.42% <100.00%> (-0.07%)	⬇️
src/anndata/tests/helpers.py	83.24% <100.00%> (-9.63%)	⬇️
src/anndata/_core/sparse_dataset.py	90.85% <50.00%> (-0.69%)	⬇️

... and 4 files with indirect coverage changes