Outdated readme

[jarrodcolburn]

The readme recommends:

• google_generative_ai which has been deprecated
• firebase_vertexai which has been discontinued (for firebase_ai?)
  

  genui/packages/genui/README.md

  Line 93 in 80beb97

  2. **Call your AI Service**: Use your preferred AI client (e.g., `google_generative_ai`, `firebase_vertexai`, or a custom HTTP client) to send the message.