refactor(github-actions): update issue labeler model to gemini-3.1-flash-lite-preview

Updates the model used for issue labeling to gemini-3.1-flash-lite-preview to address rate limiting issues and use a newer model.

TAG=agy

CONV=ff901305-db06-40a2-93f2-82f5ab684b12

Author: josephperrott

github-actions/labeling/issue/lib/issue-labeling.ts

@@ -83,7 +83,7 @@ If no area label applies, respond with "none".
 
     try {
       const response = await ai.models.generateContent({
-        model: 'gemini-2.0-flash',
+        model: 'gemini-3.1-flash-lite-preview',
         contents: prompt,
       });
       const text = (response.text || '').trim();