chore: add warning for zarr format v2

[ilan-gold]

• Begins migration to zarr v3 writing by default (will follow this up with a PR to remove zarr v2 the package as well)
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.56%. Comparing base (96a1529) to head (0d8d716).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2306      +/-   ##
==========================================
- Coverage   86.61%   84.56%   -2.06%     
==========================================
  Files          46       46              
  Lines        7224     7228       +4     
==========================================
- Hits         6257     6112     -145     
- Misses        967     1116     +149     

Files with missing lines	Coverage Δ
src/anndata/_io/zarr.py	82.79% <100.00%> (+0.77%)	⬆️

... and 7 files with indirect coverage changes