Update security notice in mutual-authentication ARM template

This PR removes a “Microsoft Confidential” security notice from a publicly published Azure Docs article.

The content is intended for internal use and is not appropriate for a public-facing document. No other changes were made.

Author: maotsuk

articles/application-gateway/mutual-authentication-arm-template.md

@@ -388,4 +388,4 @@ az deployment group create \
 
 ## Security notice
 
-This solution is classified as **Microsoft Confidential**. Please ensure you follow your organization’s security and data handling best practices when deploying and managing this solution.
+Please ensure you follow your organization’s security and data handling best practices when deploying and managing this solution.