Added agent upgrade warning

Author: johnmarco

articles/azure-arc/servers/agent-overview.md

@@ -9,6 +9,10 @@ ms.topic: conceptual
 
 The Azure Connected Machine agent enables you to manage your Windows and Linux machines hosted outside of Azure on your corporate network or other cloud providers.
 
+> [!WARNING]
+> Only Connected Machine agent versions within the last 1 year are officially supported by the product group. Customers should update to an agent version within this window.
+> 
+
 ## Agent components
 
 :::image type="content" source="media/agent-overview/connected-machine-agent.png" alt-text="Azure Connected Machine agent architectural overview." border="false":::