CI: Skip previous release testing for Ubuntu

We are hitting timeouts in the kayobe-overcloud-upgrade-ubuntu-noble job
and cannot bump the timeout higher than its current value of 3 hours.

Skip testing of the previous release for Ubuntu: we already test the
upgraded cloud, and the previous release gets tested on stable branches
anyway. This will help to reduce the job time below the timeout.

Change-Id: I041676b1505740d46324bc115e085ead84ab6d2e
Signed-off-by: Pierre Riteau <pierre@stackhpc.com>

playbooks/kayobe-overcloud-upgrade-base/run.yml

@@ -76,12 +76,14 @@
             cmd: dev/overcloud-test-vm.sh &> {{ logs_dir }}/ansible/overcloud-test-vm-pre-upgrade
             chdir: "{{ previous_kayobe_src_dir }}"
             executable: /bin/bash
+          when: ansible_facts.os_family != 'Debian'
 
         - name: Perform testing of the baremetal machines in the overcloud prior to upgrade
           shell:
             cmd: dev/overcloud-test-baremetal.sh &> {{ logs_dir }}/ansible/overcloud-test-bm-pre-upgrade
             chdir: "{{ previous_kayobe_src_dir }}"
             executable: /bin/bash
+          when: ansible_facts.os_family != 'Debian'
 
         # Upgrade Kayobe, and use it to perform an upgrade of the control plane.