MNT: Replace appdirs with platformdirs

[adswa]

The original appdirs repository is unmaintained. This change replaces it
with a currently maintained fork of the project. Closes #6164, and for now replaces #6166

[adswa]

rebased to master and added semver-minor label as per Bens suggestion

[codecov]

Codecov Report

Merging #6198 (2d57aa4) into master (615910d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6198      +/-   ##
==========================================
- Coverage   89.71%   89.71%   -0.01%     
==========================================
  Files         319      319              
  Lines       41663    41717      +54     
==========================================
+ Hits        37378    37426      +48     
- Misses       4285     4291       +6     

Impacted Files	Coverage Δ
datalad/interface/run_procedure.py	93.49% <ø> (ø)
datalad/support/external_versions.py	94.87% <ø> (ø)
datalad/cmdline/helpers.py	73.42% <100.00%> (ø)
datalad/downloaders/tests/test_providers.py	100.00% <100.00%> (ø)
datalad/interface/common_cfg.py	100.00% <100.00%> (ø)
datalad/support/cookies.py	80.88% <100.00%> (ø)
datalad/tests/test_utils.py	96.74% <0.00%> (-0.57%)	⬇️
datalad/distribution/dataset.py	96.26% <0.00%> (-0.38%)	⬇️
datalad/interface/base.py	89.51% <0.00%> (+0.05%)	⬆️
... and 1 more

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[mih]

Thx!