[MNT] use `catch22` from `conda-forge`

[Royi]

Currently, according to documentation, the installation from conda-forge is missing the catch-22 package:

Yet catch22 is available in conda-forge. See DynamicsAndNeuralSystems/catch22#35.

So the packaging can be updated on conda-forge to make the whole package work.

[Franz Király]

Thanks - I've updated the conda-forge recipe for 0.28.0, and we'll see if there are any problems.

[Franz Király]

No problems, it seems.

I've also updated the instructions in #6229, so I suppose this is resolved now?