Update load_movielens_df to return new standard field names

[mdekstrand]

This updates load_movielens_df to return the new column names instead of the legacy ones.

[codecov]

Codecov Report

Attention: Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.20%. Comparing base (acd6f5d) to head (a3e3b3a).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/movielens.py	58.82%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #572   +/-   ##
=======================================
  Coverage   90.20%   90.20%           
=======================================
  Files         103      103           
  Lines        6388     6390    +2     
=======================================
+ Hits         5762     5764    +2     
  Misses        626      626           

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