Bring back ability to make installer images for Catalina and older after commit 522fc7f

[ubihazard]

Workaround in commit 522fc7f causes installinstallmacos.py no longer being able to make installer images for Catalina, Mojave, and High Sierra.

Closes #132.

[gregneagle]

I'm going to merge this, but I'm going to simplify the logic:

If running on macOS < 15.6, use the legacy method of building the disk image (installer -pkg /path/to/dist)
If running on macOS >= 15.6, build the disk image by installing InstallAssistant.pkg. Also, print an error if the requested item does not have InstallAssistant.pkg and therefore cannot be supported under 15.6+

[gregneagle]

OK, I've tested the current code in main (as of 1275ba4) and under macOS 26.0.1 it properly builds a Tahoe installer and prints an error if you select a pre-Big Sur installer (now printing that error before it goes to the trouble of spending several minutes downloading the files). I'll leave it to others to test functionality on older OSes and building installers for pre-Big Sur.