[tlbuild] Upcoming ICU 66 release

[Nelson H. F. Beebe]

arl writes:

>> I don't want to upgrade ICU just as we are finalizing the TL image, ...

I completely understand.  I raised the issue of the ICU 66 prerelease
precisely because evolution of that library in the past has been the
cause of several woes, such as the need for C++14 or C++17 language
features.

The TL 2020 build period would be a good time to check out whether
further code/compiler changes will be needed for the 2021 release.  If
they were, we could get a year's head start, easing the job of the
next year.  Of course, we should produce final executables for 2020
using the current `stable' ICU release.

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