chore(generative_ai) Update model references for generative_ai samples

Samples under generative_ai/ have references to older models of Gemini. Please update to the current model:

🎯 Objective: Update the model being used in generative ai samples to a current one.

🛠️ To Do: Check all samples under generative_ai/ directory

🔗 Links: [Vertex AI Gen AI Models](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models)

🚧 Blockers: None