Added cast blacklist for certain transform agg funcs

[WillAyd]

• closes Transform with Count Agg against DateTime returns DateTime #19200
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19355 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19355      +/-   ##
==========================================
+ Coverage   91.54%   91.57%   +0.02%     
==========================================
  Files         150      150              
  Lines       48702    48705       +3     
==========================================
+ Hits        44585    44600      +15     
+ Misses       4117     4105      -12

Flag	Coverage Δ
#multiple	89.94% <100%> (+0.02%)	⬆️
#single	41.72% <50%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/groupby.py	92.15% <100%> (+0.01%)	⬆️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jreback]

thanks @WillAyd nice patch! keep em coming!