Readthedocs configuration: temporary revert to miniconda before minif…

…orge3 becomes available (remove Mambaforge) (#2562)

.readthedocs.yaml

@@ -7,20 +7,13 @@ version: 2
 
 # Set the version of Python and other tools you might need
 build:
-  os: ubuntu-22.04
+  os: ubuntu-lts-latest
   tools:
-    # updated and deployed from Aug 1, 2023
-    python: "mambaforge-22.9"
+    # try miniforge3 when available? see github.com/ESMValGroup/ESMValCore/issues/2557
+    # DO NOT use mambaforge-*; that is currently sunsetted
+    python: "miniconda-latest"
   jobs:
-    pre_create_environment:
-      # update mamba just in case
-      - 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
 
 # Declare the requirements required to build your docs