Update ESMValTool to v2.15 and reduce the size of the ESMValTool conda environment

[bouweandela]

Description

Use the esmvaltool-base package for a smaller conda-environment (about 2.3 GB).

See ESMValGroup/ESMValTool#4595 for documentation of the package.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

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[bouweandela]

Tests are failing because

1. the REF has not yet been updated for Update REF ENSO recipes for CMIP7 ESMValGroup/ESMValTool#4470 (esmvaltool/enso-basic-climatology diagnostic not configured for CMIP7 datasets #803)
2. Make ozone diagnostic compatible with CMIP7 data #704 was merged before the required changes landed in ESMValTool