func planUpgrade(operation *operatorv1.Operation, spec *operatorv1.UpgradeOperationSpec) *operatorv1.RuntimeTaskGroupList {
// TODO support upgrade to v1.n-1~v1.n of current kubernetes server version.
// If the current kubernetes server version is v1.n-2 which is below the target version, we need to generate a further upgrade plan
var items []operatorv1.RuntimeTaskGroup
dryRun := operation.Spec.GetTypedOperationExecutionMode() == operatorv1.OperationExecutionModeDryRun
support upgrade to v1.n-1~v1.n of current kubernetes server version.
If the current kubernetes server version is v1.n-2 which is below the target version, we need to generate a further upgrade plan
https://github.com/pacoxu/kubeadm-operator/blob/4362c623e4b00efd8612cd56cbd6b2d65a51a348/operations/upgrade.go#L28
584308d31e13a73f3a48209f7d077771027890d5