Update azidentity's BREAKING_CHANGES.md (Azure#23572)

sdk/azidentity/BREAKING_CHANGES.md

@@ -1,5 +1,15 @@
 # Breaking Changes
 
+## v1.8.0
+
+### New errors from `NewManagedIdentityCredential` in some environments
+
+`NewManagedIdentityCredential` now returns an error when `ManagedIdentityCredentialOptions.ID` is set in a hosting environment whose managed identity API doesn't support user-assigned identities. `ManagedIdentityCredential.GetToken()` formerly logged a warning in these cases. Returning an error instead prevents the credential authenticating an unexpected identity. The affected hosting environments are:
+  * Azure Arc
+  * Azure ML (when a resource or object ID is specified; client IDs are supported)
+  * Cloud Shell
+  * Service Fabric
+
 ## v1.6.0
 
 ### Behavioral change to `DefaultAzureCredential` in IMDS managed identity scenarios