Add CMIP7-compatible REF version of recipe_ecs.yml

[schlunma]

Description

Make ECS diagnostic compatible with CMIP7 data and add REF version of the recipe.

• See Overview issue on preparing REF recipes for CMIP7 #4431.
• Docs: https://esmvaltool--4465.org.readthedocs.build/en/4465/recipes/recipe_ref.html

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New or updated recipe/diagnostic

• 🧪 Recipe runs successfully
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[schlunma]

Corresponding REF PR: Climate-REF/climate-ref#671

[bouweandela]

Looks good to me!