Create mechanism for updating of Unicode data files downstreams when we want
The current mechanism for updating Unicode data files will create the following problem:
Downstream compilers/packagers will download Unicode data files ONE time (they may already have done so).
However, if they don't activate ALWAYS_UPDATE_UNICODE = yes, these files will never get updated, and they will stay on Unicode version 7.0 even if in five years Unicode is e.g. on version 12.0.
On the other hand, if they activate ALWAYS_UPDATE_UNICODE = yes (and assuming issue #10415 gets fixed), they constantly update to the latest version of Unicode. That's good for those who actually want this, but now what our current policy is.
What's missing is that we (Ruby core) can make sure downstream checkouts update to a new Unicode version when we want then to do so (as we e.g. can do for other parts that are based on Unicode data, see e.g. https://bugs.ruby-lang.org/issues/9092), without sending an email to everybody and hoping they read and follow it.
[Currently, the only solution I know will work is the one pointed out by Yui Naruse in https://bugs.ruby-lang.org/issues/10084#note-17, but I'm okay with any other solution.]