Update S3 Module README

[sj-williams]

Background

Our S3 Module contains outdated information in the README regarding instructions on external account bucket migrations. (Access keys and removed module variables). Possibly rewrite with instructions on achieving the same result via IRSA and Service Pod.

https://github.com/ministryofjustice/cloud-platform-terraform-s3-bucket?tab=readme-ov-file#cloud-platform-terraform-s3-bucket

Proposed user journey

Approach

Which part of the user docs does this impact

Communicate changes

• [ ] post for #cloud-platform-update
• [ ] Weeknotes item
• [ ] Show the Thing/P&A All Hands/User CoP
• [ ] Announcements channel

Questions / Assumptions

Definition of done

• [ ] readme has been updated
• [ ] user docs have been updated
• [ ] another team member has reviewed
• [ ] smoke tests are green
• [ ] prepare demo for the team

Reference

How to write good user stories