Deprecate classes Variable, Variables and Datasets

[schlunma]

Deprecate classes Variable, Variables and Datasets.

Closes #1943

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Checklist for technical review

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• The pull request has a descriptive title that can be used as a one line summary in the changelog
• The code is composed of functions of no more than 50 lines and uses meaningful names for variables and follows the style guide
• Documentation is available
• Please use yamllint to check that your YAML files do not contain mistakes
• (Only if really necessary) Add any additional dependencies needed for the diagnostic script to setup.py, esmvaltool/install/R/r_requirements.txt or esmvaltool/install/Julia/Project.toml (depending on the language of your script) and also to package/meta.yaml for conda dependencies (includes Python and others, but not R/Julia). Also check that the license of the dependency you want to add and any of its dependencies are compatible with Apache2.0.

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[bouweandela]

Looks good, but I realized I made a mistake with the version in which to remove the feature, see also my comment in the issue.

[bouweandela]

It would probably be good to mention that the feature is deprecated and will be removed in the docstring of the affected classes too, so it becomes visible here: https://docs.esmvaltool.org/en/latest/api/esmvaltool.diag_scripts.shared.html#esmvaltool.diag_scripts.shared.Variable

[bouweandela]

Looks good! Tested it and it works as expected.