Remove upper version bound on zarr

[flying-sheep]

zarr 3 looks like it’s still in a pretty rough state, see e.g. the bug list on cubed-dev/cubed#295

[codecov]

Codecov Report

Attention: Patch coverage is 57.57576% with 14 lines in your changes missing coverage. Please review.

Project coverage is 83.66%. Comparing base (4e43a8c) to head (c746fee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1530      +/-   ##
==========================================
- Coverage   84.36%   83.66%   -0.71%     
==========================================
  Files          35       35              
  Lines        5699     5711      +12     
==========================================
- Hits         4808     4778      -30     
- Misses        891      933      +42     

Files	Coverage Δ
src/anndata/_io/specs/methods.py	86.94% <100.00%> (-1.11%)	⬇️
src/anndata/_io/utils.py	76.47% <ø> (ø)
src/anndata/_io/zarr.py	82.50% <100.00%> (-1.25%)	⬇️
src/anndata/experimental/merge.py	78.17% <100.00%> (-13.20%)	⬇️
src/anndata/tests/helpers.py	86.19% <100.00%> (ø)
src/anndata/_core/sparse_dataset.py	91.46% <0.00%> (-1.81%)	⬇️
src/anndata/_io/specs/registry.py	95.09% <50.00%> (-1.16%)	⬇️
src/anndata/compat/__init__.py	83.19% <44.44%> (-0.29%)	⬇️