[CI] upgrade from 2025.2

Upgrade jobs should run from current branch -1

Change-Id: I1e72aa2d17f08ba070ef885623f9fb37a2bdabea
Signed-off-by: Riccardo Pittau <elfosardo@gmail.com>

Author: elfosardo

zuul.d/bifrost-jobs.yaml

@@ -198,7 +198,7 @@
     run: playbooks/ci/upgrade.yaml
     timeout: 9600
     vars:
-      upgrade_from: stable/2025.1
+      upgrade_from: stable/2025.2
       skip_upgrade:
         - openstack/diskimage-builder