Mambaforge is deprecated

[rickecon]

OG-Core and its country calibrations currently use the Mambaforge installer as the method in our GitHub Actions of installing a small-footprint version of Python to run in the GH Actions. However, Mambaforge is being deprecated and Miniforge is taking its place (see this blog post "Sunsetting Mambaforge). Key points are that Mambaforge is going away and Miniforge is taking its place. The sunsetting schedule listed in the link tells us that we will begin to experience more and more incovenient delays in our GH Actions until we make this switch. And Mambaforge will be completely gone by January 2025.

I made these changes in a PR in the UN-OG-Training repository as suggested by the setup-miniconda README.md, and they seem to work great. We need to make the same changes in OG-Core and its related country calibration repositories.

cc: @jdebacker