Update legacy_anndata.py

[Liripo]

Convert to RGB format so that colors will display correctly as RGB in napari-spatialdata.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.16%. Comparing base (c06b53f) to head (ff93890).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #320   +/-   ##
=======================================
  Coverage   55.16%   55.16%           
=======================================
  Files          26       26           
  Lines        2844     2844           
=======================================
  Hits         1569     1569           
  Misses       1275     1275           

Files with missing lines	Coverage Δ
src/spatialdata_io/converters/legacy_anndata.py	51.36% <100.00%> (ø)

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[LucaMarconato]

Thanks @Liripo, looks good to me! I resolved the merge conflicts and added a rgb parameter in case the user wants to show the channels separately.