.travis.yml: Update the jobs to Ubuntu 22.04

.travis.yml: Update the jobs to Ubuntu 22.04

According to our support policy, we'll soon drop our official support
for Ubuntu 20.04 ("Focal Fossa") in QEMU. Thus we should update the
Travis jobs now to a newer release (Ubuntu 22.04 - "Jammy Jellyfish")
for future testing. Since all jobs are using this release now, we
can drop the entries from the individual jobs and use the global
setting again.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240418101056.302103-6-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>