Error with `StatusCode` `Internal` thrown when a server is unavailable for a client running on .NET Framework 4.8 instead of `Unavailable`

### What version of gRPC and what language are you using?
Grpc.Net.Client 2.67.0 and C#

### What operating system (Linux, Windows,...) and version?
Windows 11

### What runtime / compiler are you using (e.g. .NET Core SDK version `dotnet --info`)
.NET Framework 4.8

### What did you do?
I have a GRPC server written in Java Spring Boot. I want to ensure proper retries if the server is unavailable. From what I understand, the client will retry if a retryable status code is returned. I would have thought that if the server is unavailable, I should get a status code of `StatusCode.Unavailable` but instead I get `StatusCode.Internal`. Setting `StatusCode.Internal` as a retryable status code works so far as the retries are concerned but that error is thrown for a variety of reasons that I don't want to retry on. Following is how I am setting up the channel

```c#
WinHttpHandler winHttpHandler = new WinHttpHandler();
winHttpHandler.ServerCertificateValidationCallback = TlsValidationCallback;

GrpcChannel channel = GrpcChannel.ForAddress("https://localhost:9090", new GrpcChannelOptions
{
HttpHandler = winHttpHandler,
ServiceConfig = new ServiceConfig
{
MethodConfigs =
{
new MethodConfig
{
Names = { MethodName.Default }, RetryPolicy =
new RetryPolicy
{
InitialBackoff = TimeSpan.FromSeconds(2),
MaxBackoff = TimeSpan.FromSeconds(30),
BackoffMultiplier = 1.5,
RetryableStatusCodes = { StatusCode.Internal }
}
}
}
},
MaxRetryAttempts = 5
});

Client = new HelloService.HelloServiceClient(channel);

Client.SendMessahe(new HelloRequest { Name = "hello"});
```

### What did you expect to see?

I expected to get a `StatusCode.Unavailable` when the server is not running.

### What did you see instead?

I am getting a `StatusCode.Internal` instead.

The full error message is

```
Unhandled Exception: Grpc.Core.RpcException: Status(StatusCode="Internal", Detail="Error starting gRPC call. HttpRequestException: An error occurred while sending the request. WinHttpException: Error 12029 calling WINHTTP_CALLBACK_STATUS_REQUEST_ERROR, 'A connection with the server could not be established'.", DebugException="System.Net.Http.HttpRequestException: An error occurred while sending the request.") ---> System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.Http.WinHttpException: Error 12029 calling WINHTTP_CALLBACK_STATUS_REQUEST_ERROR, 'A connection with the server could not be established'.
at System.Threading.Tasks.RendezvousAwaitable`1.GetResult()
at System.Net.Http.WinHttpHandler.d__122.MoveNext()
--- End of inner exception stack trace ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
at Grpc.Net.Client.Internal.GrpcCall`2.d__82.MoveNext() in /_/src/Grpc.Net.Client/Internal/GrpcCall.cs:line 508
--- End of inner exception stack trace ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Grpc.Net.Client.Internal.Retry.RetryCallBase`2.d__80.MoveNext() in /_/src/Grpc.Net.Client/Internal/Retry/RetryCallBase.cs:line 119
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Grpc.Net.Client.Internal.HttpClientCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request) in /_/src/Grpc.Net.Client/Internal/HttpClientCallInvoker.cs:line 153
at Grpc.Core.Interceptors.InterceptingCallInvoker.b__3_0[TRequest,TResponse](TRequest req, ClientInterceptorContext`2 ctx) in /_/src/Grpc.Core.Api/Interceptors/InterceptingCallInvoker.cs:line 53
at Grpc.Core.ClientBase.ClientBaseConfiguration.ClientBaseConfigurationInterceptor.BlockingUnaryCall[TRequest,TResponse](TRequest request, ClientInterceptorContext`2 context, BlockingUnaryCallContinuation`2 continuation) in /_/src/Grpc.Core.Api/ClientBase.cs:line 205
at Grpc.Core.Interceptors.InterceptingCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request) in /_/src/Grpc.Core.Api/Interceptors/InterceptingCallInvoker.cs:line 50
at MyRunner.RunnerService.RunnerServiceClient.ReplyRunner(HelloRequest request, CallOptions options) in my-folder\my-runner\MyRunner\obj\Debug\Protos\RunnerGrpc.cs:line 110
at MyRunner.RunnerService.RunnerServiceClient.ReplyRunner(HelloRequest request, Metadata headers, Nullable`1 deadline, CancellationToken cancellationToken) in my-folder\my-runner\MyRunner\obj\Debug\Protos\RunnerGrpc.cs:line 105
at MyRunner.RunnerCallbacks.ReplyRunner(RunnerReplyModel RunnerReplyModel) in my-folder\my-runner\MyRunner\RunnerCallbacks.cs:line 17
at Beezlabs.RPAHive.Lib.RPARunnerTemplate.RunRunner(RunnerExecutionModel RunnerExecutionModel)
at MyRunner.Program.d__0.MoveNext() in my-folder\my-runner\MyRunner\Program.cs:line 42
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
at MyRunner.Program.(String[] args)
```

### Anything else we should know about your project / environment?
`TlsValidationCallback` is a function that I am using to validate a custom certificate which is why managing my own `WinHttpHandler` is neded.

Reproduce the unavailable-server call on .NET Framework 4.8 with WinHttpHandler, then inspect Grpc.Net.Client/Internal/GrpcCall.cs around line 508 and Internal/Retry/RetryCallBase.cs around line 119. Done means a server that cannot be reached reports StatusCode.Unavailable, while unrelated Internal errors remain distinguishable for retry configuration.

Written by the indexing model from the issue text.