Switch to the .NET 7 feed (#7096)

frameworks/CSharp/aspnetcore-corert/PlatformBenchmarks/NuGet.Config

@@ -2,7 +2,7 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="dotnet-experimental" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-experimental/nuget/v3/index.json" />
+    <add key="dotnet7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7/nuget/v3/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>