Backport PR #2628 on branch 0.13.x (chore: disable zarr v2 writing (at least as far as we can control it))

[ilan-gold]

This PR:

1. leaves the setting in place and deprecates it
2. Tests that setting and then using it triggers a warning with v2
3. Leaves v3 untouched

Hopefully I got that right!

See:
https://github.com/scverse/anndata/pull/2643/changes#diff-e4676cccdf29fdbe5cfd0a760aaf30a1408f2efd19aa0889871bf505f759663b
and
https://github.com/scverse/anndata/pull/2643/changes#diff-fc69accdad779387bbaf2b952adcff99c96d09fd6416011b319b6887d5a5498e

• Closes #
• Tests added

• Release note not necessary because:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.00%. Comparing base (d38f76a) to head (5639ef4).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.13.x    #2643      +/-   ##
==========================================
- Coverage   88.01%   86.00%   -2.02%     
==========================================
  Files          49       49              
  Lines        7879     7882       +3     
==========================================
- Hits         6935     6779     -156     
- Misses        944     1103     +159     

Files with missing lines	Coverage Δ
src/anndata/_io/zarr.py	87.31% <100.00%> (+0.39%)	⬆️
src/anndata/_settings.py	100.00% <100.00%> (ø)
src/anndata/tests/helpers.py	83.24% <100.00%> (-9.08%)	⬇️

... and 8 files with indirect coverage changes

[flying-sheep]

Looks good to me. One small nitpick.