chore (Badge) : Added .NET foundation badge (#32846)

1. Found the .NET foundation badge at https://img.shields.io/badge/.NET%20Foundation-blueviolet.svg
2. Added to badge collection.

README.md

+[![.NET Foundation](https://img.shields.io/badge/.NET%20Foundation-blueviolet.svg)](https://www.dotnetfoundation.org/)
 [![Apache-2.0 Licence](https://img.shields.io/github/license/dotnet/aspnetcore?color=%230b0&style=flat-square)](https://github.com/dotnet/aspnetcore/blob/main/LICENSE.txt) [![Help Wanted](https://img.shields.io/github/issues/dotnet/aspnetcore/help%20wanted?color=%232EA043&label=help%20wanted&style=flat-square)](https://github.com/dotnet/aspnetcore/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22) [![Good First Issues](https://img.shields.io/github/issues/dotnet/aspnetcore/good%20first%20issue?color=%23512BD4&label=good%20first%20issue&style=flat-square)](https://github.com/dotnet/aspnetcore/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22)
 [![Discord](https://img.shields.io/discord/732297728826277939?style=flat-square&label=Discord&logo=discord&logoColor=white&color=7289DA)](https://aka.ms/dotnet-discord)