Remove k8s v1.25 once sylva-core moves to only supporting v1.26|v1.27|v1.28

We should drop support for k8s v1.25 once sylva-core moves to only supporting v1.26|v1.27|v1.28 in MR sylva-projects/sylva-core!1952 (merged)

# .gitlab-ci.yml

.kubeadm_settings:
  variables:
    K8S_FLAVOR: kubeadm
    KANOD_OPTIONS_K8S: "-b kubeadm"
  parallel:
    matrix:
      - K8S_VERSION:
          # renovate: datasource=github-releases depName=rancher/rke2 VersionTemplate=-rke2
          - 1.25.14-rke2r1


.rke2_settings:
  variables:
    K8S_FLAVOR: rke2
    KANOD_OPTIONS_K8S: "-b rke2_airgapped"
  parallel:
    matrix:
      - K8S_VERSION:
          # renovate: datasource=github-releases depName=rancher/rke2 VersionTemplate=-rke2
          - 1.25.14-rke2r1```

The following discussion from !177 (merged) should be addressed:

• @tmmorin started a discussion: (+1 comment)

  I think we should transition step by step:

  • introduce 1.28 and ensure everything work, including full sylva-core integration => done in !177 (merged)
  • then remove 1.25