Fixup feeds (#28144)

Move to myget-legacy so that everything is dev.azure.com

Author: mmitche

src/NuGet.Config

@@ -5,8 +5,8 @@
   </packageRestore>
   <packageSources>
     <clear />
-    <add key="myget.org dotnet-core" value="https://dotnetmygetlegacy.blob.core.windows.net/dotnet-core/index.json" />
-    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
+    <add key="myget-legacy" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/myget-legacy/nuget/v3/index.json" protocolVersion="3" />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
   </packageSources>
   <activePackageSource>
     <add key="All" value="(Aggregate source)" />

tests/src/NuGet.Config

@@ -5,7 +5,7 @@
   </packageRestore>
   <packageSources>
     <clear/>
-    <add key="myget.org dotnet-core" value="https://dotnetmygetlegacy.blob.core.windows.net/dotnet-core/index.json" />
+    <add key="myget-legacy" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/myget-legacy/nuget/v3/index.json" />
     <add key="nuget-build" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/nuget-build/nuget/v3/index.json" />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
   </packageSources>