[build] NuGet source changes to build with .NET 8 RC 1

* Remove `<packageSourceMapping>` to avoid errors like:

    Package source mapping matches found for package ID 'Microsoft.NETCore.App.Ref' are: 'dotnet-public'.
    ...
    D:\src\xamarin-android\external\Java.Interop\src\Java.Interop.Tools.Expressions\Java.Interop.Tools.Expressions.csproj error NU1102: Unable to find package Microsoft.NETCore.App.Ref with version (= 7.0.11)
    - Found 83 version(s) in dotnet-public [ Nearest version: 8.0.0-preview.1.23110.8 ]
    - Versions from dotnet-eng were not considered [D:\src\xamarin-android\Xamarin.Android.sln]

* Add feeds for dotnet/runtime 7.0.11 and 6.0.22.

* Set `$(DisableTransitiveFrameworkReferenceDownloads)` to avoid restore
  errors like:

    error NU1102: Unable to find package Microsoft.WindowsDesktop.App.Ref with version (= 7.0.11)
    error NU1102: Unable to find package Microsoft.AspNetCore.App.Ref with version (= 7.0.11)

Directory.Build.props

@@ -3,6 +3,7 @@
   <!-- Note: MUST be imported *after* $(Configuration) is set! -->
   <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <DisableTransitiveFrameworkReferenceDownloads>true</DisableTransitiveFrameworkReferenceDownloads>
     <TreatWarningsAsErrors Condition=" '$(Configuration)' == 'Release' ">true</TreatWarningsAsErrors>
     <_OutputPath>$(MSBuildThisFileDirectory)bin\Build$(Configuration)\</_OutputPath>
     <GenerateAssemblyInfo>true</GenerateAssemblyInfo>

NuGet.Config

@@ -10,15 +10,9 @@
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
     <!-- For Microsoft.DotNet.GenAPI -->
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" protocolVersion="3" />
+    <!-- Added manually for dotnet/runtime 7.0.11 -->
+    <add key="darc-pub-dotnet-runtime-a2ad4f0" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-a2ad4f03/nuget/v3/index.json" />
+    <!-- Added manually for dotnet/runtime 6.0.22 -->
+    <add key="darc-pub-dotnet-runtime-762f437" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-762f4379/nuget/v3/index.json" />
   </packageSources>
-
-  <packageSourceMapping>
-    <!-- key value for <packageSource> should match key values from <packageSources> element -->
-    <packageSource key="dotnet-public">
-      <package pattern="*" />
-    </packageSource>
-    <packageSource key="dotnet-eng">
-      <package pattern="Microsoft.DotNet.GenAPI" />
-    </packageSource>
-  </packageSourceMapping>
 </configuration>