Update "Migrating Your Legacy On-Prem SQL Server Databases to RDS"

This guide needs some small updates -

- Update mention of AWS Tools for Windows PowerShell to be AWS Tools for PowerShell, and steer readers to the modular version (so install the AWS.Tools.S3 module)
- As the PowerShell tools handle multi-part upload for you, I think that mention can be removed
- As a best practice, reduce the permissions used to just the bucket involved
- Remove mention of the AWS Tools/PowerShell in the final section, the tools aren't used there (just a standard PowerShell command prompt)

Locate the "Migrating Your Legacy On-Prem SQL Server Databases to RDS" guide and review the sections covering AWS Tools for PowerShell, multipart upload, permissions, and the final PowerShell section. Update the terminology, installation guidance, permissions scope, and unused tool reference as listed, then verify that the guide no longer describes multipart handling or tools where they are not used.

Written by the indexing model from the issue text.