docs: update RDS Proxy client_password_auth_type supported auth type

[yelinaung]

Description

The recent aws-sdk-go-v2 RDS release adds support for MYSQL_CACHING_SHA2_PASSWORD auth type.
https://github.com/aws/aws-sdk-go-v2/blob/5a964704cb2640ed57a74b9b37a53dcda7b6b7dd/service/rds/CHANGELOG.md#v1930-2024-12-16
This updates the docs regarding the RDS Proxy client_password_auth_type supported values.

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[github-actions]

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[github-actions]

Welcome @yelinaung 👋

It looks like this is your first Pull Request submission to the Terraform AWS Provider! If you haven’t already done so please make sure you have checked out our CONTRIBUTOR guide and FAQ to make sure your contribution is adhering to best practice and has all the necessary elements in place for a successful approval.

Also take a look at our FAQ which details how we prioritize Pull Requests for inclusion.

Thanks again, and welcome to the community! 😃

[ewbankkit]

LGTM 🚀.

[ewbankkit]

@yelinaung Thanks for the contribution 🎉 👏.