Most upper bounds files for transport version are for an already
branched version. For those, the upper bound must be within the base id
for that branch, and validation exists to ensure the highest id within a
branch is in the upper bound file for that branch. But for main the base
is always different. This commit adds validation to ensure that the
highest transport version id exists in _some_ upper bound file.

Note that while we could try to identify which upper bound file belongs
to main, this is tricky when validation runs in non-main branches. This
check is just as good, just a little less specific than "it should exist
in x.y upper bound file".