Migrate nuget-build feed from myget to Azure DevOps (#8305)

* Migrate nuget-build feed to AzDO

* Remove unused dotnet-core feed

Author: zivkan

NuGet.config

@@ -9,8 +9,7 @@
   </packageRestore>
   <packageSources>
     <add key="nuget" value="https://api.nuget.org/v3/index.json" />
-    <add key="nugetbuild" value="https://dotnet.myget.org/F/nuget-build/api/v3/index.json" />
-    <add key="dotnetcore" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
+    <add key="nuget-build" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/nuget-build/nuget/v3/index.json" />
   </packageSources>
   <activePackageSource>
     <add key="All" value="(Aggregate source)" />

tests/NuGet.Config

@@ -10,7 +10,7 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="nugetbuild" value="https://dotnet.myget.org/F/nuget-build/api/v3/index.json" />
+    <add key="nuget-build" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/nuget-build/nuget/v3/index.json" />
   </packageSources>
   <activePackageSource>
     <add key="All" value="(Aggregate source)" />