fixes for pandas >= 2.2.0

[dennisbader]

Summary

• fixes all breaking changes from pandas v2.2.0. Mainly the breaking changes come from deprecated frequency/offset aliases (e.g. "M" (month end) becomes "ME", "T" (minutes) becomes "min"). See here and here.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ccecc8a) 93.91% compared to head (f4a7380) 93.90%.

Files	Patch %	Lines
darts/models/forecasting/prophet_model.py	81.25%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2177      +/-   ##
==========================================
- Coverage   93.91%   93.90%   -0.02%     
==========================================
  Files         135      135              
  Lines       13387    13380       -7     
==========================================
- Hits        12573    12565       -8     
- Misses        814      815       +1     

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