feature: use plugin logger as recomended by Mkdocs (road to Mkdocs>=1.4)

[Guts]

See: https://www.mkdocs.org/dev-guide/plugins/#logging-in-plugins

[codecov]

Codecov Report

Merging #221 (c7e4310) into main (ed2e6fe) will increase coverage by 0.20%.
The diff coverage is 90.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   82.43%   82.63%   +0.20%     
==========================================
  Files          10       10              
  Lines         518      524       +6     
  Branches      108      108              
==========================================
+ Hits          427      433       +6     
  Misses         58       58              
  Partials       33       33              

Files	Coverage Δ
mkdocs_rss_plugin/constants.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/plugin.py	94.91% <100.00%> (-0.05%)	⬇️
mkdocs_rss_plugin/timezoner_pre39.py	84.61% <100.00%> (+1.28%)	⬆️
mkdocs_rss_plugin/timezoner_py39.py	84.61% <100.00%> (+1.28%)	⬆️
mkdocs_rss_plugin/util.py	72.76% <100.00%> (+0.11%)	⬆️
...lugin/integrations/theme_material_social_plugin.py	86.15% <88.88%> (+0.21%)	⬆️
mkdocs_rss_plugin/git_manager/ci.py	70.37% <71.42%> (+2.37%)	⬆️