feat(python)!: Enforce deprecation of keyword arguments as positional

[stinodego]

Ref #13525

These were deprecated in early 0.20, but I want to expedite the deprecation because:

• In the case of group_by, it blocks the semantics of using by as a column name in **named_by.
• In other cases, the deprecation was unlikely to impact many people so we might as well clean up the decorators now.

I kept the decorator implementation and added some tests, as I feel it may come in handy in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.45%. Comparing base (5eb8931) to head (4953f26).

Files	Patch %	Lines
py-polars/polars/_utils/deprecation.py	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16755      +/-   ##
==========================================
- Coverage   81.45%   81.45%   -0.01%     
==========================================
  Files        1413     1413              
  Lines      185877   185865      -12     
  Branches     2713     2701      -12     
==========================================
- Hits       151407   151395      -12     
  Misses      33979    33979              
  Partials      491      491              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.