Switch to miniforge

Due to the change in Anaconda Inc's TOS many institutions do not allow to use conda's `default` channel. Since miniconda uses `default` (e.g. for setting up the base environment and its also included in the default list of channels) we also using miniconda is forbidden.

As a drop in replacement `miniforge` should be used.

The main point is the `Makefile`, but also a few tutorials could be updated.

Start with the Makefile and search the tutorials for references to miniconda, conda's default channel, and the base environment. Update the identified setup instructions and tutorial references to use miniforge, then verify that the documented setup remains consistent.

Written by the indexing model from the issue text.