fix: Method dt.offset_by was discarding month and year info if day was included in offset for timezone-aware columns

[MarcoGorelli]

closes #21283

Previously, the logic was:

• let mut new_t = t
• then, when adding years / months, increment new_t
• then, when adding days, increment new_t

The issue was that for the last step, it was starting the calculation from t rather than from new_t, which is why the linked issue reports that if 'd' is included, then years and months get discarded

I don't think new_t is necessary at all - t can just get incremented at each step, so like this there's no risk of accidentally incrementing the wrong variable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.89%. Comparing base (5e51218) to head (9b4b5ed).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21291      +/-   ##
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+ Coverage   79.82%   79.89%   +0.07%     
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  Files        1596     1596              
  Lines      228565   228562       -3     
  Branches     2608     2608              
==========================================
+ Hits       182462   182620     +158     
+ Misses      45507    45346     -161     
  Partials      596      596              

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