Readthedocs builds still use Mambaforge

[valeriupredoi]

We retired Mambaforge in #3774 and #3778 - everywhere in ESMValTool, apart from the RTD configuration, since at the mo I don't think they support a Miniforge3 variant, and going back to Miniconda is just silly, see https://github.com/readthedocs/readthedocs.org/issues/11690

We reverted to miniconda-latest in #3785 and, as @bouweandela found out, that configuration is taking significantly less to build the environment (since mamba is the default solver for miniconda), so, we may want to keep it permanently; if we decide to test with Miniforge3, when it becomes available, the previous commands in the readthedocs.yml file were useful:

# Set the version of Python and other tools you might need
build:
  os: ubuntu-22.04
  tools:
    # replace when miniforge3 when available
    # DO NOT use mambaforge-*; that is currently sunsetted
    python: "miniconda-latest"  # miniforge3
  jobs:
   # Put these back when we have Miniforge3 from RTD
   #   pre_create_environment:
   #     # update mamba
   #     - mamba update --yes --quiet --name=base mamba 'zstd=1.5.2'
   #     - mamba --version
   #     - mamba list --name=base
    post_create_environment:
   #     - conda run -n ${CONDA_DEFAULT_ENV} mamba list
   #     # use conda run executable wrapper to have all env variables
   #     - conda run -n ${CONDA_DEFAULT_ENV} mamba --version
      - conda run -n ${CONDA_DEFAULT_ENV} pip install . --no-deps

So that we don't keep commented out code floating about, I will remove those calls, and maybe put them back when using Miniforge3.

[valeriupredoi]

the conversation in that issue suggests they will need quite a bit of time to move to Miniforge3; in the meantime, we'll most probably experience the "brownouts" via RTD not building our docs, and that very frequently in November, so I suggest we move to the (old style) Miniconda container in our RTD config, in the meantime, what do @ESMValGroup/technical-lead-development-team folk think?