Comment out ORT-Nightly feed in test app NuGet.config (#16762)

### Description
<!-- Describe your changes. -->
Comment out ORT-Nightly feed in NuGet.config to see if that makes the
Secure Supply Chain Analysis CI step happy.

Add info to readme on manually adding feed and using it.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

csharp/tools/MauiModelTester/NuGet.config

@@ -6,9 +6,11 @@
   <packageSources>
     <clear />
     <add key="NuGet Official" value="https://api.nuget.org/v3/index.json" />
+    <!-- Uncomment the following line to use packages from the ORT nightly feed
     <add key="ORT-Nightly" value="https://aiinfra.pkgs.visualstudio.com/PublicPackages/_packaging/ORT-Nightly/nuget/v3/index.json" />
+    -->
   </packageSources>
   <disabledPackageSources>
     <clear />
   </disabledPackageSources>
-</configuration>
+</configuration>

csharp/tools/MauiModelTester/ReadMe.md

@@ -16,13 +16,24 @@ Resources\Raw\test_data\test_data_set_0
 The MAUI application will read the model and test data from those locations and should need no other changes to be able
 to execute the model.
 
-NOTE: The project uses builds from the nightly feed to keep things simple.
+The project uses builds from the nightly feed by default to keep things simple.
 
 If it was part of the main ONNX Runtime C# solution we'd have to
   - add the ORT nightly feed to the top level nuget.config
     - this potentially adds confusion about where nuget packages come from in unit tests
   - keep updating the referenced nightly packages so they remain valid so the complete solution builds in the CI
 
+You will need to manually add the ORT-Nightly feed to the packageSources section of the nuget.config in this directory.
+  - `<add key="ORT-Nightly" value="https://aiinfra.pkgs.visualstudio.com/PublicPackages/_packaging/ORT-Nightly/nuget/v3/index.json" />`
+  - This feed isn't allowed in the checked in nuget.config
+
+If you need to update the ORT packages used by the app to the latest nightly:
+- In Visual Studio, Tools -> Nuget Package Manager -> Manage NuGet Packages for Solution...
+- Make sure 'Include prerelease' is checked
+- Set Package Source to ORT-Nightly
+- Update Microsoft.ML.OnnxRuntime, Microsoft.ML.OnnxRuntime.Managed and Microsoft.ML.OnnxRuntime.Extensions to the
+latest build.
+
 ## Testing C# or native code changes
 
 If you have new code to test the easiest way is to run the nuget packaging pipeline on