Update REF recipe recipe_ref_fire.yml for CMIP7 datasets

[jlenh]

Description

The recipe recipe_ref_fire.yml needs some updates with respect to CMIP7 datasets.

Previous issue #4343 and PR #4423 already dealt with some initial differences, notably for the tasmax variable, but a few fixes are still needed.

Data available for testing alongside the notebook to produce the files /home/b/b381312/projects/esmvaltool_cmip7_dummy

• Closes Update REF recipe recipe_ref_fire.yml to support CMIP7 datasets #4478
• Link to documentation:

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