Update CmdstanPy converter

[ahartikainen]

Description

Checklist

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2287.org.readthedocs.build/en/2287/

[codecov]

Codecov Report

Merging #2287 (bc153df) into main (c2d968f) will increase coverage by 0.10%.
The diff coverage is 32.12%.

@@            Coverage Diff             @@
##             main    #2287      +/-   ##
==========================================
+ Coverage   86.78%   86.89%   +0.10%     
==========================================
  Files         122      122              
  Lines       12460    12628     +168     
==========================================
+ Hits        10814    10973     +159     
- Misses       1646     1655       +9     

Files	Coverage Δ
arviz/data/io_cmdstanpy.py	43.27% <32.12%> (+27.39%)	⬆️

[ahartikainen]

@OriolAbril this is quite minimal change and still works with older models.
For test changes, we can probably create a new PR?

[ahartikainen]

I will quickly check cases for 1.1.0 so it will work too.

[ahartikainen]

So now it should support

<0.9.67
<1.0.0
<1.20
1.2.0+