[firebase_vertexai] Add a "Deprecated" notice to the README on pub.dev

[htsuruo]

What feature would you like to see?

Add a "Deprecated" notice to the README of the firebase_vertexai on pub.dev like google_generative_ai package.

Reason

I found the PR for creating firebase_ai.
#17332

It looks like firebase_ai is the package copied by firebase_vertexai and both are quite similar.

And more, the documentation is stated following:

Firebase AI Logic was formerly called "Vertex AI in Firebase" with the plugin firebase_vertexai.
https://firebase.google.com/docs/flutter/setup?platform=ios#available-plugins

The developers may be a bit confused about which Dart package I should use for implementing generative AI.

I understand there have been a lot of changes since the Google I/O '25 announcements and the introduction of Firebase AI Logic. Here's my current understanding:

Before:

• google_generative_ai: This package directly used the Gemini Developer API. This meant developers had to store the Gemini API Key directly in the app, making it suitable only for prototypes, not production applications.
• firebase_vertexai: This was an SDK for Vertex AI for Firebase. It supported Firebase App Check, which made it suitable for production apps.

After:

• firebase_ai: This is the SDK for Firebase AI Logic, which combines google_generative_ai and firebase_vertexai. google_generative_ai has been deprecated and replaced to this, and the package is implemented by exporting and reusing the original firebase_vertexai implementation.
• firebase_vertexai: This package seems to have remained unchanged.

In fact, firebase_ai supports using both the Gemini Developer API and Vertex AI.