Remove deprecated callback argument from preprocessor load function

[bouweandela]

Description

Remove the deprecated argument callback from the preprocessor function load.

Upgrade instructions: use esmvalcore.dataset.Dataset.load() instead.

Closes #1800

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[codecov]

Codecov Report

Merging #2207 (fded02d) into main (a6f257a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2207      +/-   ##
==========================================
- Coverage   93.14%   93.14%   -0.01%     
==========================================
  Files         237      237              
  Lines       12841    12831      -10     
==========================================
- Hits        11961    11951      -10     
  Misses        880      880              

Files	Coverage Δ
esmvalcore/_recipe/recipe.py	99.01% <ø> (-0.01%)	⬇️
esmvalcore/dataset.py	100.00% <100.00%> (ø)
esmvalcore/preprocessor/__init__.py	95.22% <100.00%> (-0.02%)	⬇️
esmvalcore/preprocessor/_io.py	86.80% <100.00%> (-0.31%)	⬇️

[schlunma]

Thanks Bouwe, everything looks good to me. I successfully ran some example recipes with this 👍

[valeriupredoi]

nice cheers 🍺