travis.yml: Update the s390x jobs to Ubuntu Focal

travis.yml: Update the s390x jobs to Ubuntu Focal

QEMU will soon drop the support for Ubuntu 18.04, so let's update
the Travis jobs that were still using this version to 20.04 instead.

While we're at it, also remove an obsolete comment about Ubuntu
Xenial being the default for our Travis jobs.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220221153423.1028465-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220225172021.3493923-15-alex.bennee@linaro.org>