OpenSCAD.download.recipe URL Change

[dcoobs]

It appears that GitHub Pages sites stopped redirecting *github.com to *github.io yesterday (April 15), which broke the URL used by the OpenSCAD download recipe.
https://github.blog/changelog/2021-01-29-github-pages-will-stop-redirecting-pages-sites-from-github-com-after-april-15-2021/

Looking at the notes in the download recipe, it seems there were previously issues with the SSL cert for https://openscad.org/appcast.xml, but that appears to have been fixed now. The recipe can likely be switched to using the direct URL rather than indirectly via the GitHub Pages site.

I've opened another issue with the developer to update the Sparkle XML file.