gitlab: upgrade the job definition for s390x to 20.04

gitlab: upgrade the job definition for s390x to 20.04

The new s390x machine has more of everything including the OS. As
18.04 will soon be going we might as well get onto something moderately
modern.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220225172021.3493923-16-alex.bennee@linaro.org>