Update nuget.config for best practices (#3677)

Following new guidance on best practices, this shouldn't effect anything, so if the CI runs we should be all good.

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<!-- - Bugfix -->
<!-- - Feature -->
<!-- - Code style update (formatting) -->
- Refactoring (no functional changes, no api changes)
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Author: msftbot[bot]

nuget.config

@@ -1,8 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
     <add key="AzureLatest" value="https://pkgs.dev.azure.com/dotnet/WindowsCommunityToolkit/_packaging/WindowsCommunityToolkit-MainLatest/nuget/v3/index.json" protocolVersion="3" />
     <add key="MUX-Shared" value="https://pkgs.dev.azure.com/ms/microsoft-ui-xaml/_packaging/MUX-Shared/nuget/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>