Restore 16.0.0 changelog for AI Logic

Author: emilypgoogle

firebase-ai/CHANGELOG.md

@@ -66,5 +66,13 @@
     free tier offering.
   - Using the Firebase AI SDK with the Vertex AI Gemini API is still generally available (GA).
 
+If you're using the old `firebase-vertexai`, we recommend
+[migrating to `firebase-ai`](/docs/ai-logic/migrate-to-latest-sdk) because all new development and
+features will be in this new SDK.
+
+- [feature] **Preview:** Added support for specifying response modalities in `GenerationConfig`
+  (only available in the new `firebase-ai` package). This includes support for image generation
+  using [specific Gemini models](/docs/vertex-ai/models).
+
 Note: This feature is in Public Preview, which means that it is not subject to any SLA or
 deprecation policy and could change in backwards-incompatible ways.