Update AR To AWS-SDK v3

[JamieB-gu]

More information in guardian/pinboard#249

[rhiannareechaye]

@JamieB-gu I'm struggling to understand this ticket - is this a must?

[Georges-GNM]

AR currently uses AWS-SDK v2, which is being phased out and so we will want to upgrade to v3. See AWS info:
https://docs.aws.amazon.com/sdkref/latest/guide/maint-policy.html
https://github.com/aws/aws-sdk-js#version-2x-support
Should investigate a bit further to confirm when v2 will reach end of life