you can disable new Sigv4 signing:

using Amazon;
using Amazon.Runtime;
using Amazon.S3;

var request = new PutObjectRequest
{
    FilePath = @"/path/file.txt",
    BucketName = "theBucket",
    DisablePayloadSigning = true,           // sigv4 is introduced in the new s3 sdk
    DisableDefaultChecksumValidation = true // need to disable it until you migrate
};

var response = await s3Client.PutObjectAsync(request);

Console.WriteLine("ETag: {0}", response.ETag);

https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html