[Fabric Bot] Update Attestation Contacts (Azure#28449)

The focus of these changes is to update the service contacts for Attestation, mirroring recent changes
to CODEOWNERS.

Author: jsquire

.github/fabricbot.json

@@ -1529,7 +1529,8 @@
               "Attestation"
             ],
             "mentionees": [
-              "anilba06"
+              "anilba06",
+              "gkostal"
             ]
           },
           {