fix: Incorrect result for top_k/bottom_k when input is sorted

[lukemanley]

closes #21260

import polars as pl

pl.Series([1, 2, 3, 4]).sort().top_k(3)

# BEFORE

# shape: (3,)
# Series: '' [i64]
# [
#         1
#         2
#         3
# ]

# AFTER

# shape: (3,)
# Series: '' [i64]
# [
#         2
#         3
#         4
# ]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.87%. Comparing base (9077aa4) to head (a6acde9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21264      +/-   ##
==========================================
- Coverage   79.87%   79.87%   -0.01%     
==========================================
  Files        1596     1596              
  Lines      228469   228468       -1     
  Branches     2607     2607              
==========================================
- Hits       182487   182482       -5     
- Misses      45386    45390       +4     
  Partials      596      596              

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