Fix violin plot deprecation warnings from ggplot2 4.0

[jgabry]

Branches on ggplot2 version to use correct quantile syntax for violin plots.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (edffd04) to head (aa1a309).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #410   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          35       35           
  Lines        5784     5792    +8     
=======================================
+ Hits         5704     5712    +8     
  Misses         80       80           

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