Remove the deprecated option use_legacy_supplementaries

[bouweandela]

Description

Remove the deprecated option use_legacy_supplementaries, scheduled for removal in v2.10.0 (see #1609 (comment)).

Upgrade advice

The recommended upgrade procedure is to remove use_legacy_supplementaries from config-user.yml (if it was there) and remove any mention of fx_variables from the recipe. If automatically defining the required supplementary variables does not work, define them in the variable or (additional_)datasets section as described in the documentation.

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[codecov]

Codecov Report

Merging #2202 (c57cb32) into main (10bd776) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2202      +/-   ##
==========================================
- Coverage   93.14%   93.06%   -0.09%     
==========================================
  Files         237      237              
  Lines       12831    12654     -177     
==========================================
- Hits        11951    11776     -175     
+ Misses        880      878       -2     

Files	Coverage Δ
esmvalcore/_recipe/recipe.py	98.99% <100.00%> (-0.02%)	⬇️
esmvalcore/_recipe/to_datasets.py	100.00% <100.00%> (ø)
esmvalcore/config/_config_validators.py	97.45% <ø> (-0.10%)	⬇️
esmvalcore/preprocessor/__init__.py	95.23% <ø> (ø)
esmvalcore/preprocessor/_supplementary_vars.py	97.67% <ø> (-0.39%)	⬇️

[valeriupredoi]

is ESMValCore so light now that it can ✈️ ? I'd see about CMIP7 project, bud 😁

[bouweandela]

Thanks for reviewing!