depr: Rename aggregate_function to agg_function

[nikaltipar]

#25467

This seems to fit the naming convention better. We have .agg() or even agg_list parameters in some methods.

[orlp]

Marked do not merge as no decision has been made yet: #25467.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.16%. Comparing base (6a76be8) to head (adf86ce).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/src/polars/lazyframe/frame.py	73.33%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25477      +/-   ##
==========================================
- Coverage   82.16%   82.16%   -0.01%     
==========================================
  Files        1728     1728              
  Lines      240146   240189      +43     
  Branches     3031     3031              
==========================================
+ Hits       197320   197350      +30     
- Misses      42045    42058      +13     
  Partials      781      781              

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