Tweak instructions for upgrading Utopia

# required for using the latest Utopia.

#

function(register_models_with_frontend)

    #

    # NOTE If changing something here, also check the doc/dev/versioning.rst

    #      file, making sure that the instructions match each other.

    #

    message(FATAL_ERROR

        "It looks like you have updated Utopia.\n"

        "This error is to inform you about the changes needed to get your "

        "where this error message comes) to the updated function name "

        "`register_models_with_utopya`. After having done that, this error "

        "message will no longer appear.\n"

        "3) Run `cmake ..` again.\n"

        "3) Remove the CMake cache file (`build/CMakeCache.txt`) in your "

        "models repository or remove your whole `build` directory to avoid "

        "carrying over outdated variables.\n"

        "4) Run `cmake ..` in your models repository.\n"

        "This should get your project running again with the latest version "

        "of Utopia. However, there may be further steps necessary to update "

        "your model plots and tests. "

Upgrading from ``v1``

^^^^^^^^^^^^^^^^^^^^^

.. NOTE If changing something here, also check whether the information in

        cmake/modules/UtopiaUtopyaCalls.cmake is still valid.

        If not, also update it there.

To upgrade Utopia to its latest version, simply pull the latest changes:

.. code-block:: bash

    In the ``metadata`` node, you can put in some descriptive info about your models, e.g. a list of `authors` or a short `description`.

    *Hint:* You can also have a look at Utopia's ``.utopya-project.yml`` file to see more possible entries.

  - Delete the ``build/CMakeCache.txt`` file in your models repository to avoid carrying over outdated variables.

    If you get an error in the next step, try deleting the whole ``build`` directory and creating it anew.

  - Now, run ``cmake ..`` again.

- Furthermore, you *may* need to update ``import`` statements in your Python model tests or model plots because the utopya and dantro package structures changed.