Remove deprecated k= argument alias from Top-N metrics

[mdekstrand]

This removes the deprecated k= argument from top-N metrics in favor of n=, for consistency across LensKit that list lengths are n.

Closes #889.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.59%. Comparing base (dc16ad7) to head (baf8613).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1073      +/-   ##
==========================================
+ Coverage   89.48%   89.59%   +0.10%     
==========================================
  Files         231      187      -44     
  Lines       15867    13192    -2675     
==========================================
- Hits        14199    11819    -2380     
+ Misses       1668     1373     -295     

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