.Net: Reduce visibility of two fields from public to private (#3354)

### Motivation and Context

Two properties were inadvertently made public and should be private

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/SemanticKernel.Core/Functions/SemanticFunction.cs

@@ -144,9 +144,9 @@ public string ToString(bool writeIndented)
     private static readonly JsonSerializerOptions s_toStringIndentedSerialization = new() { WriteIndented = true };
     private readonly ILogger _logger;
     private IAIServiceSelector? _serviceSelector;
-    public List<AIRequestSettings>? _modelSettings;
+    private List<AIRequestSettings>? _modelSettings;
     private readonly Lazy<FunctionView> _view;
-    public IPromptTemplate _promptTemplate { get; }
+    private readonly IPromptTemplate _promptTemplate;
 
     private static async Task<string> GetCompletionsResultContentAsync(IReadOnlyList<ITextResult> completions, CancellationToken cancellationToken = default)
     {