Previously builds were done with Ubuntu 16.04, and now we are using
18.04. Thus the generated binaries will no longer work for
Ubuntu 16.04 and Debian 9 users, both of which are outside of their
support window and aren't supported by Abseil. RHEL users are
unaffected, as the binaries already didn't work on RHEL 7 and they will
remain working with RHEL 8. FWIW, Ubuntu 18.04 will leave its support
window in June.