Update NuGet package icon to new Microsoft Foundry Agent Framework logo (microsoft#6812)

Replace dotnet/nuget/icon.png with the new Microsoft Foundry Agent Framework color logo (resized to 128x128, the NuGet-recommended icon size). Source: docs/assets/PNG/Microsoft Foundry Agent Framework - Color.png.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: chetantoshniwal