Read the Docs Iris is pinned to 2.4 because of lack of recent Afterburner conda-forge package

In the Read the Docs conda environment in `.readthedocs_conda_env.yml`, Iris has been pinned to version 2.4. This is because the most recent version of metoffice-afterburner available on conda-forge is 1.3.2b1.post4, which is limited to Iris<3.0. Once a more recent version of Afterburner has been released to conda-forge then this pin of Iris for Rtds should be removed.

Check .readthedocs_conda_env.yml and confirm whether a newer metoffice-afterburner package is available on conda-forge. Once the dependency is released, remove the Iris 2.4 pin and verify that the Read the Docs conda environment resolves successfully.

Written by the indexing model from the issue text.