update change log

Author: VinayGuthal

firebase-ai/CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
-- [feature] Added support for code execution.
+- [feature] Added support for code execution. Marked the public constructors for
+  `ExecutableCodePart` and `CodeExecutionResultPart` as deprecated.
 - [feature] Introduced `MissingPermissionsException`, which is thrown when the necessary permissions
   have not been granted by the user.
 - [feature] Added helper functions to `LiveSession` to allow developers to track the status of the