Update Gemini template [Go Web App] to new SDK

## Current template:
https://studio.firebase.google.com/new/gemini

### Template Options:
Environment: Go Web App
Use LangChain: False

## Updates Needed

- [ ] Migrate generative AI SDK to the new [Go SDK](https://pkg.go.dev/google.golang.org/genai)
- [ ] Update any code examples and README

