Rasterio 1.2.0 supports Python 3.6-3.9 and Numpy 1.15+ per #1813 and https://numpy.org/neps/nep-0029-deprecation_policy.html.

I propose we treat GDAL the same as Numpy, meaning support releases made within 24 months of our expected release date, with a minimum of 3 minor versions. That would mean supporting GDAL versions 3.2, 3.1, 3.0, 2.4, and 2.3 since 2.3.3 was released within 24 months of rasterio 1.2a1.

Make sure README, setup, pyproject, CI config is up to date.