[services] Replace deprecated aws_config::from_env()

Summary:
The aws_config::from_env() was deprecated in favor of defaults() which accepts a BehaviorVersion param.
I set the behavior explicitly to the latest available. We could also use latest() but I wanted to avoid it in case of breaking changes.

Depends on D12879

Test Plan: Made sure that services talk to localstack

Reviewers: varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12880