chore: Update model in Go Quickstart tests (#1631)

## Description

---
Updating the Gemini model from 1.5-flash to 2.0-flash in the Go
Quickstart tests for LangChain and Genkit samples.

This PR fixes the failing tests in
https://github.com/googleapis/genai-toolbox/pull/1572

## PR Checklist

---
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before writing your code! That way we can discuss the change, evaluate
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- [x] Ensure the tests and linter pass
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🛠️ Fixes #<issue_number_goes_here>

docs/en/getting-started/quickstart/go/genkit/quickstart.go

@@ -48,7 +48,7 @@ func main() {
 	// Initialize Genkit
 	g, err := genkit.Init(ctx,
 		genkit.WithPlugins(&googlegenai.GoogleAI{}),
-		genkit.WithDefaultModel("googleai/gemini-1.5-flash"),
+		genkit.WithDefaultModel("googleai/gemini-2.0-flash"),
 	)
 	if err != nil {
 		log.Fatalf("Failed to init genkit: %v\n", err)

docs/en/getting-started/quickstart/go/langchain/quickstart.go

@@ -59,7 +59,7 @@ func main() {
 	ctx := context.Background()
 
 	// Initialize the Google AI client (LLM).
-	llm, err := googleai.New(ctx, googleai.WithAPIKey(genaiKey), googleai.WithDefaultModel("gemini-1.5-flash"))
+	llm, err := googleai.New(ctx, googleai.WithAPIKey(genaiKey), googleai.WithDefaultModel("gemini-2.0-flash"))
 	if err != nil {
 		log.Fatalf("Failed to create Google AI client: %v", err)
 	}