Update old S3 links to point at Maven downloads

[headius]

There are still many links from older JRuby versions to old S3 locations that no longer exist, and we continue to get third-party reports of broken links:

https://twitter.com/growse/status/1160215604550393856

We should rewrite all links possible to point at Maven locations, and for those artifacts that have disappeared permanently (Windows installer builds for example) we should remove those links and/or add a note indicating they were destroyed in the Great Purging.