Remove net5 feeds (#41620)

#41593

Author: ViktorHofer

NuGet.config

@@ -15,8 +15,6 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
-    <add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
-    <add key="dotnet5-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5-transport/nuget/v3/index.json" />
     <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
     <add key="dotnet6-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6-transport/nuget/v3/index.json" />
     <!-- Used for the Rich Navigation indexing task -->