Compatibility with Dask 2025.4

[trexfeathers]

This has already been discussed on #6438, but I wanted to have a PR targetting v3.12.x as this makes the Git history less confusing.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.80%. Comparing base (8962e3b) to head (263704a).
⚠️ Report is 2 commits behind head on v3.12.x.

Files with missing lines	Patch %	Lines
lib/iris/fileformats/netcdf/saver.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           v3.12.x    #6451   +/-   ##
========================================
  Coverage    89.80%   89.80%           
========================================
  Files           90       90           
  Lines        23752    23752           
  Branches      4418     4418           
========================================
  Hits         21331    21331           
  Misses        1672     1672           
  Partials       749      749           

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[bjlittle]

@trexfeathers Awesome, thanks!

LGTM 👍