MIGRATION ISSUE: Provide example for migrating from Event Listener to Middleware Stack for a common use case

[trivikr]

Pre-Migration Checklist

• I've read the Migration Guide.
• I've reviewed the upgrading notes and major version differences mentioned in UPGRADING.md.
• I've checked AWS Forums and StackOverflow for similar migration issues.

Which JavaScript Runtime is this issue in?

Node.js (includes AWS Lambda)

AWS Lambda Usage

• Yes, my application is running on AWS Lambda.
• No, my application is not running on AWS Lambda.

Describe the Migration Issue

The https://github.com/aws/aws-sdk-js-v3/blob/main/UPGRADING.md provides link to Middleware Stack blog post which explains how a middleware should be written. But there's no example of migration from v2 event listener to v3 middleware.

Code Comparison

No response

Observed Differences/Errors

N/A. This request is to add example of v2 event listener to v3 middleware in UPGRADING.md

Additional Context

No response