Update AWS SDK version in order to use AWS IAM role via web identity token file

When you use this app in AWS EKS (Elastic Kubernetes Service) and you want to attach an IAM role to the pod it is not possible to use the native [feature of EKS](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html).
This feature requires AWS SDK 3.15 or higher, currently, this app is using [this library](https://www.npmjs.com/package/@aws-sdk/client-s3-node) which is pretty old. [See more in](https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts-minimum-sdk.html).

I suggest to roll back the changes made in https://github.com/hackmdio/codimd/commit/2fe10a78b73c357acb612c33ac925b49afed0526 and use the official SDK