chore: deprecate AnnData.*_keys methods

[flying-sheep]

• Closes Deprecate <thing>_keys methods #2101
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.70%. Comparing base (c5184ae) to head (038c091).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (31.70%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2102      +/-   ##
==========================================
- Coverage   31.79%   31.70%   -0.10%     
==========================================
  Files          46       46              
  Lines        7085     7090       +5     
==========================================
- Hits         2253     2248       -5     
- Misses       4832     4842      +10     

Files with missing lines	Coverage Δ
src/anndata/_core/anndata.py	23.87% <100.00%> (+0.49%)	⬆️

... and 2 files with indirect coverage changes