Deprecate save parameter of plots

[Zethson]

• Closes Saving plots results in a prefixed filename, regardless of sc.settings.figdir #1508
• Tests included
• Deprecates the save parameter of plotting functions
• Suggests to use sc.pl.plot(show=False).figure.savefig() instead
• Uses a Futurewarning instead of a Deprecationwarning in line with what has been done so far

TODO

• Discuss what to do with settings.figdir
• Deprecate there or in savefig_or_show

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.72%. Comparing base (d04ab0a) to head (934906c).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3675      +/-   ##
==========================================
+ Coverage   76.56%   76.72%   +0.16%     
==========================================
  Files         115      115              
  Lines       12402    12389      -13     
==========================================
+ Hits         9496     9506      +10     
+ Misses       2906     2883      -23     

Files with missing lines	Coverage Δ
src/scanpy/plotting/_anndata.py	85.55% <ø> (ø)
src/scanpy/plotting/_docs.py	100.00% <ø> (ø)
src/scanpy/plotting/_preprocessing.py	86.79% <ø> (ø)
src/scanpy/plotting/_stacked_violin.py	86.13% <ø> (ø)
src/scanpy/plotting/_tools/__init__.py	75.88% <ø> (ø)
src/scanpy/plotting/_tools/paga.py	66.60% <ø> (ø)
src/scanpy/plotting/_tools/scatterplots.py	83.13% <ø> (ø)
src/scanpy/plotting/_utils.py	64.22% <100.00%> (+3.63%)	⬆️

... and 1 file with indirect coverage changes