feat: add upgrade plan for k3s server on release channel for v1.28

manifests/system-upgrade/k3s-upgrade-v1.28.yaml

@@ -0,0 +1,26 @@
+# Cluster has only k3s-server nodes, no need for k3s-agent plan
+apiVersion: upgrade.cattle.io/v1
+kind: Plan
+metadata:
+  name: k3s-server-v1.28
+  namespace: system-upgrade
+  labels:
+    k3s-upgrade: server
+spec:
+  concurrency: 1 # Batch size (roughly maps to maximum number of unschedulable nodes)
+  channel: https://update.k3s.io/v1-release/channels/v1.28
+  nodeSelector:
+    matchExpressions:
+      - {key: k3s-upgrade, operator: Exists}
+      - {key: k3s-upgrade, operator: NotIn, values: ["disabled", "false"]}
+      - {key: k3os.io/mode, operator: DoesNotExist}
+      - {key: node-role.kubernetes.io/control-plane, operator: Exists}
+  serviceAccountName: system-upgrade
+  cordon: true
+  drain:
+    force: false
+    ignoreDaemonSets: true
+    deleteLocalData: true
+    skipWaitForDeleteTimeout: 180
+  upgrade:
+    image: rancher/k3s-upgrade