More flexibility in the miniconda version

[mstriebs]

I had the problem, that the latest release used python 3.9.5 which was incompatible with some python-packages I needed. With this adjustment one could select any miniconda version in the provided repository choose older python versions

[ViralBShah]

@stevengj This one looks good to merge.

[stevengj]

I'm worried that merging this will break people's existing preference files, since it changes the meaning of MINICONDA_VERSION.