docs: Update reference docs

[henryiii]

3.9 (and in the last binary release, 3.8) also have universal installers. Nobody should be building with 10.6 anymore, 10.9 is the minimum in most cases, so the table and description will need a bit of an update. The last 3.7 installers are 10.9+ only, and the last 3.6 installers include both. (The past EOL 3.5 was the last one to only ship 10.6+ only).

The universal installers for 3.8 are 11+ only, weirdly, unlike 3.9+, which works 10.9+ even if they say 11.

Originally posted by @henryiii in https://github.com/scikit-build/scikit-build/issues/593#issuecomment-944346842