Allow all valid legend_loc options

[flying-sheep]

• Closes sc.pl.umap "legend_loc" does not incorporate matplotlib.legend options #761, closes sc.pl.umap(... legend_loc='upper right') does not work #2322, closes Description of "legend_loc" option in scanpy.pl.embedding is inaccurate or not implemented yet. #2229, closes Matplotlib default legend locations do not work with PCA plotting  #2267
• Tests included or not required because:

• Release notes not necessary because:

Also

• documents the parameter better, see Better document legend_loc=None for embedding plots #1502
• remove duplicated tests for embedding plots with continuous variables: legend_loc does nothing there (yet)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.52%. Comparing base (4269ed2) to head (6801394).
Report is 56 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3163   +/-   ##
=======================================
  Coverage   76.52%   76.52%           
=======================================
  Files         109      109           
  Lines       12480    12483    +3     
=======================================
+ Hits         9550     9553    +3     
  Misses       2930     2930           

Files with missing lines	Coverage Δ
src/scanpy/plotting/_anndata.py	85.02% <100.00%> (ø)
src/scanpy/plotting/_docs.py	100.00% <ø> (ø)
src/scanpy/plotting/_tools/paga.py	66.98% <ø> (ø)
src/scanpy/plotting/_tools/scatterplots.py	86.99% <100.00%> (+0.05%)	⬆️
src/scanpy/plotting/_utils.py	55.86% <100.00%> (+0.08%)	⬆️