backend/s3: Upgrade to `aws-sdk-go-base/v2`

[gdavison]

The AWS provider will be upgrading to aws-sdk-go-base/v2 as part of v4.0. Upgrading to aws-sdk-go-base/v2 will provide enhancements to the AWS authentication process and add consistency to the configuration of both the backend and the provider.

Note: Because aws-sdk-go-base/v2 changes the authentication flow to be more in-line with the default flow for the AWS CLI and SDKs, this will be a multi-stage process over several releases of Terraform to prevent breaking changes. It will also require changes to aws-sdk-go-base/v2 itself.

[FireballDWF]

Resolving is expected to resolve the current problem where an S3onOutposts bucket can't be used as the S3 bucket for backend/s3