Fix boxplot labels for Matplotlib 3.11

[dnerini]

See https://app.readthedocs.org/projects/pysteps/builds/33904697/

Fix the documentation build with matplotlib 3.11, where the deprecated labels argument was removed from Axes.boxplot. Set the x-axis labels separately to preserve compatibility with older matplotlib versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.64%. Comparing base (45824f2) to head (d9f443d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #567   +/-   ##
=======================================
  Coverage   84.64%   84.64%           
=======================================
  Files         170      170           
  Lines       15095    15095           
=======================================
  Hits        12777    12777           
  Misses       2318     2318           

Flag	Coverage Δ
unit_tests	84.64% <ø> (ø)

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[dnerini]

build successful in https://app.readthedocs.org/projects/pysteps/builds/33920845/