docs: update gemini model (#2990)

## Description

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- [ ] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

Author: twishabansal

docs/en/documentation/getting-started/quickstart/js/genkit/quickstart.js

@@ -30,7 +30,7 @@ export async function main() {
         apiKey: process.env.GEMINI_API_KEY || GOOGLE_API_KEY
       })
     ],
-    model: googleAI.model('gemini-2.0-flash'),
+    model: googleAI.model('gemini-2.5-flash'),
   });
 
   const toolboxTools = await toolboxClient.loadToolset("my-toolset");

docs/en/documentation/getting-started/quickstart/js/langchain/quickstart.js

@@ -25,7 +25,7 @@ const queries = [
 
 export async function main() {
   const model = new ChatGoogleGenerativeAI({
-    model: "gemini-2.0-flash",
+    model: "gemini-2.5-flash",
   });
 
   const client = new ToolboxClient("http://127.0.0.1:5000");

docs/en/documentation/getting-started/quickstart/js/llamaindex/quickstart.js

@@ -39,7 +39,7 @@ export async function main() {
 
   // Initialize LLM
   const llm = gemini({
-    model: GEMINI_MODEL.GEMINI_2_0_FLASH,
+    model: GEMINI_MODEL.GEMINI_2_5_FLASH_LATEST,
     apiKey: GOOGLE_API_KEY,
   });