Set decode timedelta to False since it will default to this in later xarray version [all tests ci]

[ctuguinay]

Resolves /~https://github.com/OSOceanAcoustics/echopype/actions/runs/13098048680/job/36542811253?pr=1429#step:15:944 found in #1439:

Warning: In a future version of xarray decode_timedelta will default to False rather than None. To silence this warning, set decode_timedelta to True, False, or a 'CFTimedeltaCoder' instance.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.22%. Comparing base (9f56124) to head (c3b308e).
Report is 180 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
==========================================
+ Coverage   83.52%   85.22%   +1.69%     
==========================================
  Files          64       72       +8     
  Lines        5686     6590     +904     
==========================================
+ Hits         4749     5616     +867     
- Misses        937      974      +37     

Flag	Coverage Δ
unittests	85.22% <ø> (+1.69%)	⬆️

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[ctuguinay]

@leewujung Really short change, ready for review

[leewujung]

I was curious what would happen if we set decode_timedelta = True. Turned out that the "MatlabTimeStamp" variable gets messed up:

[leewujung]

Looks great, thanks! Merging now.