[release-1.6] CNPG image change broke upgrades from 1.4/1.5

in https://gitlab.com/sylva-projects/sylva-core/-/jobs/15488942686, which is a "☁️capo 🚀rke2 🎬upgrade-from-release-1.4 🛠️ha 🐧suse" pipeline, we see that a major version upgrade of postgres is attempted

      ╰┄╴Pod/keycloak/cnpg-keycloak-2-major-upgrade-hv2xz InProgress                        Pod is in the Pending phase
         ├┄╴┬┄┄[Conditions]
         ┆  ├┄╴PodReadyToStartContainers                  True
         ┆  ├┄╴Initialized                                True
         ┆  ├┄╴Ready                                      False      ContainersNotReady     containers with unready status: [major-upgrade]
         ┆  ├┄╴ContainersReady                            False      ContainersNotReady     containers with unready status: [major-upgrade]

it fails

same issue has been observed in Orange downstream CI

I suspect that it's related to the change of image that has been done in !8565 (merged)

in https://sylva-projects.gitlab.io/-/sylva-core/-/jobs/15463866777/artifacts/image_refs_management_2695221583.json we see that before this MR, release-1.6 was using "ghcr.io/cloudnative-pg/postgresql:18.1-system-trixie", so we shouldn't have upgraded this to 18.3