update image generation sample to use gemini-2.5-flash-image-preview (#2715)

Author: thatfiredev

firebase-ai/app/src/main/java/com/google/firebase/quickstart/ai/FirebaseAISamples.kt

@@ -191,11 +191,11 @@ val FIREBASE_AI_SAMPLES = listOf(
         editingMode = EditingMode.STYLE_TRANSFER
     ),
     Sample(
-        title = "Gemini 2.0 Flash - image generation",
-        description = "Generate and/or edit images using Gemini 2.0 Flash",
+        title = "Gemini 2.5 Flash - image generation (preview)",
+        description = "Generate and/or edit images using Gemini 2.5 Flash Image Preview",
         navRoute = "chat",
         categories = listOf(Category.IMAGE),
-        modelName = "gemini-2.0-flash-preview-image-generation",
+        modelName = "gemini-2.5-flash-image-preview",
         initialPrompt = content {
             text(
                 "Hi, can you create a 3d rendered image of a pig " +