[NuGet.config] Remove feeds we don't need anymore. (#21100)

NuGet.config

@@ -32,36 +32,13 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
     <add key="Dotnet arcade" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
-    <!-- We've dropped support for .NET 6, but let's keep the feed in for a little while longer to make sure it's dropped throughout our entire stack and the lack of this feed doesn't break our build. -->
-    <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
     <add key="dotnet7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7/nuget/v3/index.json" />
     <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <!-- dotnet8-transport is required for Microsoft.DotNet.Cecil -->
     <add key="dotnet8-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8-transport/nuget/v3/index.json" />
     <add key="macios-dependencies" value="https://pkgs.dev.azure.com/xamarin/public/_packaging/macios-dependencies/nuget/v3/index.json" />
     <add key="xamarin-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/xamarin-impl/nuget/v3/index.json" />
     <add key="local-tests-feed" value="tests/.nuget/packages" />
-    <!-- These feeds are from main (6.0.4xx runtime branch) to get 6.0.9 versions of numerous .NET 6 packages. It should be possible to remove this after a while. -->
-    <add key="darc-pub-dotnet-emsdk-3f6c45a" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-emsdk-3f6c45a2/nuget/v3/index.json" />
-    <add key="darc-pub-dotnet-runtime-531f715" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-531f715f/nuget/v3/index.json" />
-    <!-- Need 6.0.10 feeds too -->
-    <add key="darc-pub-dotnet-runtime-8c6bcad" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-8c6bcad1/nuget/v3/index.json" />
-    <!-- And a 6.0.11 feed -->
-    <add key="darc-pub-dotnet-runtime-f2980ae" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-f2980ae1/nuget/v3/index.json" />
-    <!-- And a 6.0.12 feed -->
-    <add key="darc-pub-dotnet-runtime-15d285f" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-15d285f3/nuget/v3/index.json" />
-    <!-- Add a 6.0.13 feed -->
-    <add key="darc-pub-dotnet-runtime-de84cf9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-d5edd29b-1/nuget/v3/index.json" />
-    <!-- Add a 6.0.14 feed -->
-    <add key="darc-pub-dotnet-runtime-2e45bc7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-2e45bc7a/nuget/v3/index.json" />
-    <!-- Add a 7.0.11 feed -->
-    <add key="darc-pub-dotnet-runtime-0ece505" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-0ece5051/nuget/v3/index.json" />
-    <!-- Add a 7.0.17 feed -->
-    <add key="darc-pub-dotnet-runtime-21f293d" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-21f293d8/nuget/v3/index.json" />
-    <!-- Add a 7.0.18 feed -->
-    <add key="darc-pub-dotnet-runtime-b6e8c6a" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-b6e8c6a6/nuget/v3/index.json" />
-    <!-- Add a 7.0.19 feed -->
-    <add key="darc-pub-dotnet-runtime-397f9ea" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-397f9eac/nuget/v3/index.json" />
     <!-- Add a 7.0.20 feed -->
     <add key="darc-pub-dotnet-runtime-fd36db5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-fd36db5f/nuget/v3/index.json" />
   </packageSources>