release: handle new major release of kubernetes

[tormath1]

When there is a new major of Kubernetes there is no previousPatches in the schedule.yaml file:

- release: 1.30
  releaseDate: 2024-04-17
  next:
    release: 1.30.1
    cherryPickDeadline: 2024-05-10
    targetDate: 2024-05-15
  maintenanceModeStartDate: 2025-04-28
  endOfLifeDate: 2025-06-28

In this case, we extend the current release with .0 to build the full version.

Closes: #61

Testing done

$ KBS_VERS=$(curl -fsSL --retry-delay 1 --retry 60 --retry-connrefused \
                --retry-max-time 60 --connect-timeout 20  \
                https://raw.githubusercontent.com/kubernetes/website/main/data/releases/schedule.yaml \
                | yq -r '.schedules[] | .previousPatches[0] // (.release = .release + ".0") | .release' \
                | awk '{print "kubernetes-v"$1}')
$ KBS_VERS_ARRAY=(${KBS_VERS})
$ printf "%s\n" "${KBS_VERS_ARRAY[@]}"
kubernetes-v1.30.0
kubernetes-v1.29.3
kubernetes-v1.28.8
kubernetes-v1.27.12