Update inject API reference samples to V4

[normj]

Description

The custom samples that are injected into the API reference docs were not updated V4. In fact they were so out date they were referencing V2 of the SDK.

I went through and updated the samples project to target .NET 8, use async/await and add null checks.

Motivation and Context

#3869

Testing

Dry Run Success: DRY_RUN-e2890913-0dad-4fef-a554-b1ebd871104d

Download the API docs from Catapult and confirmed the updated samples made it into the docs. Here is the snippet that was called on in the GitHub issue for being out of date to now use async/await and have the null check.

<div class="sectionbody">
<example><p>
          This example shows how to list all objects in a bucket.
        </p><h4 class="csharp-code-sample-title">ListObjects sample</h4><pre class="brush: csharp">

// Create a client
AmazonS3Client client = new AmazonS3Client();

// List all objects
ListObjectsRequest listRequest = new ListObjectsRequest
{
    BucketName = &quot;amzn-s3-demo-bucket&quot;,
};

ListObjectsResponse listResponse;
do
{
    // Get a list of objects
    listResponse = await client.ListObjectsAsync(listRequest);

    if (listResponse.S3Objects != null)
    {
        foreach (S3Object obj in listResponse.S3Objects)
        {
            Console.WriteLine(&quot;Object - &quot; + obj.Key);
            Console.WriteLine(&quot; Size - &quot; + obj.Size);
            Console.WriteLine(&quot; LastModified - &quot; + obj.LastModified);
            Console.WriteLine(&quot; Storage class - &quot; + obj.StorageClass);
        }
    }

    // Set the marker property
    listRequest.Marker = listResponse.NextMarker;
} while (listResponse.IsTruncated.GetValueOrDefault());

                
</pre></example>

[ashishdhingra]

Looks good