Upstream has had an incident and lots a bunch of stuff [1].
The new site no longer hosts these tarballs, instead they now
publish them via GitHub.
They've only imported the latest point versions of each major
version, which means we're forced to update from ICU 59.1 to
59.2, which at least on the surface seems to not affect us.
Ref https://trac.wildfiregames.com/ticket/5729.
[1] http://blog.unicode.org/2020/04/technical-alert-unicode-technical.html
Depends on D2716.