fix(Redis authn&authz): Add username field

[Meggielqk]

https://emqx.atlassian.net/browse/EDOC-573?atlOrigin=eyJpIjoiMWQ5NjMwZjNkYzgyNGE1MjkzYjQ3OTcxMmM0ZDNhNmUiLCJwIjoiaiJ9

[Copilot]

Pull Request Overview

This PR adds documentation for the username field in Redis authentication and authorization configurations, supporting Redis ACL (Access Control List) functionality introduced in Redis 6.0.

Key Changes:

• Added comprehensive documentation for the new username field in Redis authentication and authorization
• Reorganized configuration sections to distinguish between Dashboard configuration and configuration file methods
• Updated instructions for password field to clarify behavior with and without username

Reviewed Changes

Copilot reviewed 4 out of 7 changed files in this pull request and generated 6 comments.

File	Description
zh_CN/access-control/authz/redis.md	Added Dashboard configuration section and username field documentation for Chinese authorization docs
zh_CN/access-control/authn/redis.md	Added username field documentation and restructured password field description for Chinese authentication docs
en_US/access-control/authz/redis.md	Added username field documentation and reorganized configuration instructions for English authorization docs
en_US/access-control/authn/redis.md	Added username field documentation with Redis ACL details for English authentication docs

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}