Reduce number of minimum draw in warning message

[aloctavodia]

The minimum number of draws needed for a reliable computation of ESS and R-hat is ~100 per chain. This number should be lower on the many-chains regime. But I am not aware of a rule of thumb that we can use to compute the minimum value as a function of the number of chains. Hence this is a reasonable default value.

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📚 Documentation preview 📚: https://pymc--7163.org.readthedocs.build/en/7163/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c3e0d86) 92.29% compared to head (c18fa97) 92.29%.
Report is 1 commits behind head on main.

Additional details and impacted files

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##             main    #7163   +/-   ##
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  Coverage   92.29%   92.29%           
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  Files         101      101           
  Lines       16947    16947           
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  Hits        15642    15642           
  Misses       1305     1305           

Files	Coverage Δ
pymc/sampling/mcmc.py	87.71% <100.00%> (ø)