[tlbuild] Upcoming ICU 66 release

[Karl Berry]

    ICU 66 (scheduled for release in 2020 March) 

How unfortunate. I don't want to upgrade ICU just as we are finalizing
the TL image, "low impact" or not. Nor do I want to push back our
release since it needs to mesh with the mailing of the first DTK issue
for the year.

I think this happened last year, too. What we currently have in TL is
63. (March 2018 maybe?). Looking at http://site.icu-project.org/, I'm
hinking I should import 65 into TL (though 64.2 is tempting), and assume
we will not have 66 for TL20. I.e., always a year behind their latest
release. That doesn't sound so awful, actually :).

Comments/suggestions welcome ... --thanks, karl.