DOC: Update doc strings to show pd.Panel has been deprecated

[topper-123]

Add lines to doc string to show that Panel has been deprecated.

[codecov]

Codecov Report

Merging #18956 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18956      +/-   ##
==========================================
+ Coverage   91.57%   91.59%   +0.02%     
==========================================
  Files         150      150              
  Lines       48964    48964              
==========================================
+ Hits        44838    44850      +12     
+ Misses       4126     4114      -12

Flag	Coverage Δ
#multiple	89.96% <ø> (+0.02%)	⬆️
#single	41.13% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.68% <ø> (ø)	⬆️
pandas/core/panel.py	96.85% <ø> (ø)	⬆️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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[jreback]

thanks. note that #18934 is covering this generally.