We upgrade dependents of upgraded formulae to make sure that an upgrade
doesn't break anything. However, this reasoning applies only when a
dependent is bottled, since attempting a source build is just as likely
to break things, if not more.

I've opted not to restrict this to, say, users only on outdated versions
of macOS in order to cleanly handle other cases where this change should
also apply: Linux, or current versions of macOS in a non-default prefix.