chore: stale AnnData.concatenate references

[rwbaber]

This PR fixes stale references to AnnData.concatenate, which was removed from anndata (scverse/anndata#2370) and is no longer available in the hosted intersphinx inventory. These stale references were causing the Read the Docs build to fail.

Changes:

• Replace broken Sphinx AnnData.concatenate method links with plain literal text.
• Update the Harmony time-series example and test to use anndata.concat.
• Preserve the previous unique-index behavior with index_unique="-".

• Closes #
• Tests included or not required because:

• Release notes not necessary because: this is just a fix for stale historical references caused by an upstream anndata API removal.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.87%. Comparing base (af6de2e) to head (860ac8f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4201      +/-   ##
==========================================
- Coverage   79.87%   79.87%   -0.01%     
==========================================
  Files         121      121              
  Lines       12932    12932              
==========================================
- Hits        10330    10329       -1     
- Misses       2602     2603       +1     

Flag	Coverage Δ
hatch-test.low-vers	78.97% <ø> (-0.04%)	⬇️
hatch-test.pre	79.82% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/external/pp/_mnn_correct.py	22.22% <ø> (ø)
src/scanpy/external/tl/_harmony_timeseries.py	28.57% <ø> (ø)

... and 1 file with indirect coverage changes