docs: expand macOS platform support coverage

docs: expand macOS platform support coverage

We initially claimed to only support the most recent macOS
release, which is currently 10.15. Our Travis CI, however,
is validating 10.14.4 / XCode 10.3.

For almost all of our other platforms, we support multiple
releases to some degree. This change brings macOS in line
with other long life distros, covering the most recent &
most recent but one for a 2 year overlap. With this docs
change our CI is now actually testing our minimum version.

Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>