fix: update topics in pubspec.yaml for firebase_ai (#17759)

Author: mit-mit

packages/firebase_ai/firebase_ai/pubspec.yaml

@@ -4,9 +4,9 @@ version: 3.3.0
 homepage: https://firebase.google.com/docs/vertex-ai/get-started?platform=flutter
 topics:
   - firebase
-  - vertexai
-  - gemini
   - generative-ai
+  - gemini
+  - imagen
 
 # Explicit about the supported platforms.
 platforms: