OSDOCS-10509: RHEL deprecation notice in OTA docs

Author: skopacz1

updating/updating_a_cluster/updating-cluster-rhel-compute.adoc

@@ -14,6 +14,11 @@ To do: Remove this comment once 4.13 docs are EOL.
 
 You can perform minor version and patch updates on an {product-title} cluster. If your cluster contains {op-system-base-full} machines, you must take additional steps to update those machines.
 
+[IMPORTANT]
+====
+The use of {op-system-base} compute machines on {product-title} clusters has been deprecated and will be removed in a future release.
+====
+
 == Prerequisites
 
 * Have access to the cluster as a user with `admin` privileges.