Deprecate visium

[flying-sheep]

• Closes deprecate/remove read spatial and plotting spatial from scanpy. #2331
• Tests included or not required because:

• Release notes not necessary because:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.66%. Comparing base (1cd5a00) to head (6f91b2b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3407   +/-   ##
=======================================
  Coverage   76.65%   76.66%           
=======================================
  Files         112      112           
  Lines       12931    12935    +4     
=======================================
+ Hits         9912     9916    +4     
  Misses       3019     3019           

Files with missing lines	Coverage Δ
src/scanpy/datasets/_datasets.py	100.00% <100.00%> (ø)
src/scanpy/plotting/_tools/scatterplots.py	86.75% <100.00%> (+0.06%)	⬆️
src/scanpy/readwrite.py	76.88% <100.00%> (+0.05%)	⬆️