Switch dendrogram implementation to aggregate

[flying-sheep]

• Closes sc.tl.dendrogram no longer(?) works in backed mode #3199
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.10%. Comparing base (c6766d7) to head (3a9b3f6).

Files	Patch %	Lines
src/scanpy/tools/_dendrogram.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3204      +/-   ##
==========================================
- Coverage   76.63%   74.10%   -2.54%     
==========================================
  Files         109      109              
  Lines       12533    12537       +4     
==========================================
- Hits         9605     9290     -315     
- Misses       2928     3247     +319     

Files	Coverage Δ
src/scanpy/_compat.py	78.26% <100.00%> (+0.48%)	⬆️
src/scanpy/tools/_dendrogram.py	65.30% <83.33%> (-21.66%)	⬇️

... and 7 files with indirect coverage changes