Add missing changelog for Microsoft.Dashboard .NET release (Azure#51816)

* reset

* update changelog

* update changelog

Author: joshelton1

sdk/grafana/Azure.ResourceManager.Grafana/CHANGELOG.md

@@ -1,14 +1,10 @@
 # Release History
 
-## 1.2.0-beta.3 (Unreleased)
-
-### Features Added
-
-### Breaking Changes
+## 1.2.0-beta.3 (2025-08-07)
 
 ### Bugs Fixed
 
-### Other Changes
+- Added missing long running operation header for Grafana Update operation.
 
 ## 1.2.0-beta.2 (2025-08-04)