Fix legend labels in plot_ppc to reflect prior or posterior.

[jedludlow]

Description

Legend labels were not tracking the selected group. In many instances
they were hardcoded to "Posterior ...".

Addresses #1966.

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1967 (523f47a) into main (a67d81c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1967   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         114      114           
  Lines       12308    12308           
=======================================
  Hits        11180    11180           
  Misses       1128     1128           

Impacted Files	Coverage Δ
arviz/plots/backends/matplotlib/ppcplot.py	97.61% <100.00%> (ø)

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[OriolAbril]

Thanks for the PR! Do you want to add it to the changelog?

[jedludlow]

I added another commit which updates the CHANGELOG.