Adding a note about flutter_ai_toolkit replacing previous AI packages (#12616)

This kinda sorta fixes #12476.
Very soon, @csells is updating these pages as part of his current work.

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Parker Lougheed <[email protected]>

Author: 3 people

src/content/ai-toolkit/index.md

@@ -10,6 +10,15 @@ next:
 
 Hello and welcome to the Flutter AI Toolkit!
 
+:::note
+These pages are now out of date. They will be
+updated soon but, in the meantime, be aware that the
+`google_generative_ai` and `vertexai_firebase` packages
+are deprecated and replaced with [`package:firebase_ai`][].
+:::
+
+[`package:firebase_ai`]: {{site.pub-pkg}}/firebase_ai
+
 The AI Toolkit is a set of AI chat-related widgets that make
 it easy to add an AI chat window to your Flutter app.
 The AI Toolkit is organized around an abstract