.Net: Removed Kernel events (#9748)

### Motivation and Context

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Related: #9499

Removed the usage of obsolete Kernel Events in favor of Filters as part
of Filters graduation process.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [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 😄

Author: dmytrostruk

dotnet/samples/Concepts/Filtering/Legacy_KernelHooks.cs

@@ -37,61 +37,6 @@ public async Task ObservabilityWithFiltersAsync()
         Console.WriteLine(await kernel.InvokePromptAsync("How many days until Christmas? Explain your thinking.", new(settings)));
     }
 
-    /// <summary>
-    /// Shows how to observe the execution of a <see cref="KernelPlugin"/> instance with hooks.
-    /// </summary>
-    [Fact]
-    [Obsolete("Events are deprecated in favor of filters.")]
-    public async Task ObservabilityWithHooksAsync()
-    {
-        // Create a kernel with OpenAI chat completion
-        IKernelBuilder kernelBuilder = Kernel.CreateBuilder();
-        kernelBuilder.AddOpenAIChatCompletion(
-                modelId: TestConfiguration.OpenAI.ChatModelId,
-                apiKey: TestConfiguration.OpenAI.ApiKey);
-
-        kernelBuilder.Plugins.AddFromType<TimeInformation>();
-
-        Kernel kernel = kernelBuilder.Build();
-
-        // Handler which is called before a function is invoked
-        void MyInvokingHandler(object? sender, FunctionInvokingEventArgs e)
-        {
-            Console.WriteLine($"Invoking {e.Function.Name}");
-        }
-
-        // Handler which is called before a prompt is rendered
-        void MyRenderingHandler(object? sender, PromptRenderingEventArgs e)
-        {
-            Console.WriteLine($"Rendering prompt for {e.Function.Name}");
-        }
-
-        // Handler which is called after a prompt is rendered
-        void MyRenderedHandler(object? sender, PromptRenderedEventArgs e)
-        {
-            Console.WriteLine($"Rendered prompt: {e.RenderedPrompt}");
-        }
-
-        // Handler which is called after a function is invoked
-        void MyInvokedHandler(object? sender, FunctionInvokedEventArgs e)
-        {
-            if (e.Result.Metadata is not null && e.Result.Metadata.ContainsKey("Usage"))
-            {
-                Console.WriteLine("Token usage: {0}", e.Result.Metadata?["Usage"]?.AsJson());
-            }
-        }
-
-        // Add the handlers to the kernel
-        kernel.FunctionInvoking += MyInvokingHandler;
-        kernel.PromptRendering += MyRenderingHandler;
-        kernel.PromptRendered += MyRenderedHandler;
-        kernel.FunctionInvoked += MyInvokedHandler;
-
-        // Invoke the kernel with a prompt and allow the AI to automatically invoke functions
-        OpenAIPromptExecutionSettings settings = new() { FunctionChoiceBehavior = FunctionChoiceBehavior.Auto() };
-        Console.WriteLine(await kernel.InvokePromptAsync("How many days until Christmas? Explain your thinking.", new(settings)));
-    }
-
     /// <summary>
     /// A plugin that returns the current time.
     /// </summary>

dotnet/samples/GettingStarted/Step7_Observability.cs

@@ -240,16 +240,6 @@ public async Task<FunctionResult> InvokeAsync(
             // Quick check for cancellation after logging about function start but before doing any real work.
             cancellationToken.ThrowIfCancellationRequested();
 
-            // Invoke pre-invocation event handler. If it requests cancellation, throw.
-#pragma warning disable CS0618 // Events are deprecated
-            var invokingEventArgs = kernel.OnFunctionInvoking(this, arguments);
-#pragma warning restore CS0618 // Events are deprecated
-
-            if (invokingEventArgs?.Cancel is true)
-            {
-                throw new OperationCanceledException($"A {nameof(Kernel)}.{nameof(Kernel.FunctionInvoking)} event handler requested cancellation before function invocation.");
-            }
-
             var invocationContext = await kernel.OnFunctionInvocationAsync(this, arguments, functionResult, isStreaming: false, async (context) =>
             {
                 // Invoking the function and updating context with result.
@@ -259,22 +249,6 @@ public async Task<FunctionResult> InvokeAsync(
             // Apply any changes from the function filters context to final result.
             functionResult = invocationContext.Result;
 
-            // Invoke the post-invocation event handler. If it requests cancellation, throw.
-#pragma warning disable CS0618 // Events are deprecated
-            var invokedEventArgs = kernel.OnFunctionInvoked(this, arguments, functionResult);
-#pragma warning restore CS0618 // Events are deprecated
-
-            if (invokedEventArgs is not null)
-            {
-                // Apply any changes from the event handlers to final result.
-                functionResult = new FunctionResult(this, invokedEventArgs.ResultValue, functionResult.Culture, invokedEventArgs.Metadata ?? functionResult.Metadata);
-            }
-
-            if (invokedEventArgs?.Cancel is true)
-            {
-                throw new OperationCanceledException($"A {nameof(Kernel)}.{nameof(Kernel.FunctionInvoked)} event handler requested cancellation after function invocation.");
-            }
-
             logger.LogFunctionInvokedSuccess(this.PluginName, this.Name);
 
             this.LogFunctionResult(logger, this.PluginName, this.Name, functionResult);
@@ -370,16 +344,6 @@ public async IAsyncEnumerable<TResult> InvokeStreamingAsync<TResult>(
                 // Quick check for cancellation after logging about function start but before doing any real work.
                 cancellationToken.ThrowIfCancellationRequested();
 
-                // Invoke pre-invocation event handler. If it requests cancellation, throw.
-#pragma warning disable CS0618 // Events are deprecated
-                var invokingEventArgs = kernel.OnFunctionInvoking(this, arguments);
-#pragma warning restore CS0618 // Events are deprecated
-
-                if (invokingEventArgs?.Cancel is true)
-                {
-                    throw new OperationCanceledException($"A {nameof(Kernel)}.{nameof(Kernel.FunctionInvoking)} event handler requested cancellation before function invocation.");
-                }
-
                 FunctionResult functionResult = new(this, culture: kernel.Culture);
 
                 var invocationContext = await kernel.OnFunctionInvocationAsync(this, arguments, functionResult, isStreaming: true, (context) =>