Migrate AWS SDK for JavaScript v2 to v3

[mauricio-oa]

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When running the existing version of the aws-api-gateway-cli-test, the below warning it thrown:

Authenticating with User Pool
(node:34471) NOTE: The AWS SDK for JavaScript (v2) is in maintenance mode.
 SDK releases are limited to address critical bug fixes and security issues only.

Please migrate your code to use AWS SDK for JavaScript (v3).
For more information, check the blog post at https://a.co/cUPnyil
(Use `node --trace-warnings ...` to show where the warning was created)

Reference: https://aws.amazon.com/blogs/developer/announcing-end-of-support-for-aws-sdk-for-javascript-v2/