Update .NET version requirement in getting-started.md from 8.0 to 9.0

Author: willibrandon

docs/getting-started.md

@@ -13,7 +13,7 @@ When you're using AI coding assistants like Cursor AI or Claude, they work best
 ## Quick Start (5 Minutes)
 
 ### Prerequisites
-- .NET 8.0 or later
+- .NET 9.0 or later
 - For semantic search: Azure OpenAI account (optional but recommended)
 - Git