Accept that we can't get versions for such locale names for now.  Users
will need to specify the newer language tag format to enable the
collation versioning feature.  It's not clear that we can do automatic
conversion from the old style to the new style reliably enough for this
purpose.

Unfortunately, this means that collation versioning probably won't work
for the default collation unless you provide something like en-US at
initdb or CREATE DATABASE time (though, for reasons not yet understood,
it does seem to work on some systems).  It'd be nice to find a better
solution, or document this quirk if we settle on it, but this should
unbreak the 3 failing build farm animals in the meantime.
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAEepm%3D0uEQCpfq_%2BLYFBdArCe4Ot98t1aR4eYiYTe%3DyavQygiQ%40mail.gmail.com