Update ms.service to "microsoft-defender-for-identity"

This replaces "azure-advanced-threat-protection", which is no longer valid. I'm applying "microsoft-defender-for-identity" based on this value in other (but not all) articles in this repo.

Author: garycentric

ATPDocs/index.yml

@@ -6,7 +6,7 @@ metadata:
   title: Microsoft Defender for Identity documentation
   description: Microsoft Defender for Identity cloud service helps protect your enterprise hybrid environments from multiple types of advanced targeted cyber attacks and insider threats.
   services: service
-  ms.service: azure-advanced-threat-protection
+  ms.service: microsoft-defender-for-identity
   ms.subservice: subservice
   ms.topic: landing-page
   ms.collection: M365-security-compliance