Update download page post-stable/15 branch

The main branch is now 16.0-CURRENT; and we now have a 15.0-STABLE
branch.

We don't have 15.0-STABLE downloads quite yet but they should arrive
in the next week; I'd rather have some temporary 404s than forget to
add the 15.0-STABLE section later.

Sponsored by: https://www.patreon.com/cperciva