Remove preview version badge from README (#822)

* Initial plan

* Remove preview version badge from README

Co-authored-by: joseharriaga <45773732+joseharriaga@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: joseharriaga <45773732+joseharriaga@users.noreply.github.com>

Author: Copilot

README.md

@@ -1,6 +1,6 @@
 # OpenAI .NET API library
 
-[![NuGet stable version](https://img.shields.io/nuget/v/openai.svg)](https://www.nuget.org/packages/OpenAI) [![NuGet preview version](https://img.shields.io/nuget/vpre/openai.svg)](https://www.nuget.org/packages/OpenAI/absoluteLatest)
+[![NuGet stable version](https://img.shields.io/nuget/v/openai.svg)](https://www.nuget.org/packages/OpenAI)
 
 The OpenAI .NET library provides convenient access to the OpenAI REST API from .NET applications.