Add changelog entry

Author: daymxn

FirebaseAI/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+- [deprecated] All Imagen models are deprecated and will shut down as early as June 2026.
+  As a replacement, you can [migrate your apps to use Gemini Image models (the "Nano Banana" models)](https://firebase.google.com/docs/ai-logic/imagen-models-migration).
+
 # 12.12.0
 - [fixed] Updated `GenerativeModelSession` to handle intermediate JSON decoding
   failures when streaming structured data. (#15960)