Create chat-retention-policy

Updating preview message for better clarity

Author: kperla97

articles/communication-services/includes/chat-retention-policy

@@ -0,0 +1,6 @@
+ms.custom: private_preview_retention_policy
+---
+> [!IMPORTANT]
+> The data retention policy described in this section is currently in public preview.
+> This preview version is provided without a service-level agreement, and we don't recommend it for production workloads. Certain features might not be supported or might have constrained capabilities.
+> For more information, see [Supplemental Terms of Use for Microsoft Azure Previews](https://azure.microsoft.com/support/legal/preview-supplemental-terms/).