Update docs as AKS cluster auto upgrade is now GA

Ref: Azure/AKS#1303 (comment)

website/docs/r/kubernetes_cluster.html.markdown

@@ -84,12 +84,10 @@ In addition, one of either `identity` or `service_principal` blocks must be spec
 
 * `automatic_channel_upgrade` - (Optional) The upgrade channel for this Kubernetes Cluster. Possible values are `patch`, `rapid`, `node-image` and `stable`. Omitting this field sets this value to `none`.
 
-!> **Note:** Cluster Auto-Upgrade will update the Kubernetes Cluster (and it's Node Pools) to the latest GA version of Kubernetes automatically - please [see the Azure documentation for more information](https://docs.microsoft.com/en-us/azure/aks/upgrade-cluster#set-auto-upgrade-channel-preview).
+!> **Note:** Cluster Auto-Upgrade will update the Kubernetes Cluster (and its Node Pools) to the latest GA version of Kubernetes automatically - please [see the Azure documentation for more information](https://docs.microsoft.com/en-us/azure/aks/upgrade-cluster#set-auto-upgrade-channel).
 
 -> **Note:** Cluster Auto-Upgrade only updates to GA versions of Kubernetes and will not update to Preview versions.
 
-~> **NOTE:** Auto upgrade channel is in Public Preview - more information and details on how to opt into the Preview [can be found in this article](https://docs.microsoft.com/en-us/azure/aks/upgrade-cluster#set-auto-upgrade-channel-preview).
-
 * `addon_profile` - (Optional) A `addon_profile` block as defined below.
 
 * `api_server_authorized_ip_ranges` - (Optional) The IP ranges to allow for incoming traffic to the server nodes.