Update logging of loglevel changes

[dweindl]

Since #3150, there are log messages related to changing loglevels during module imports.

I think, we only want notifications for changed loglevels for pre-existing loggers.

When we are changing the loglevel from debug to something higher, we don't want to see yet another debug message. Therefore, log after applying the new loglevel.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.72%. Comparing base (22b0965) to head (c97fa1f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3158      +/-   ##
==========================================
- Coverage   77.36%   76.72%   -0.64%     
==========================================
  Files         317      317              
  Lines       20629    20631       +2     
  Branches     1483     1483              
==========================================
- Hits        15959    15829     -130     
- Misses       4662     4794     +132     
  Partials        8        8              

Flag	Coverage Δ
cpp	70.45% <100.00%> (+<0.01%)	⬆️
cpp_python	37.15% <100.00%> (+<0.01%)	⬆️
petab	47.17% <100.00%> (+<0.01%)	⬆️
python	68.91% <100.00%> (+<0.01%)	⬆️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/logging.py	79.72% <100.00%> (+0.56%)	⬆️

... and 4 files with indirect coverage changes

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