tuning: recommend external authentication

[moio]

Description

Clarify that in high scale scenarios an external authentication system is really needed for proper operation.

Note that the local authentication provider is already discouraged also in general:

In most cases, you should use an external authentication service over local authentication, as external authentication allows user management from a central location. However, you may want a few local authentication users for managing Rancher under rare circumstances, such as if your external authentication provider is unavailable or undergoing maintenance.

https://ranchermanager.docs.rancher.com/how-to-guides/new-user-guides/authentication-permissions-and-global-configuration/authentication-config

This is about making it clearer for high scale scenarios.

Comments

This applies to all versions current and future - for the time being I changed all files identically and opened this PR against the main branch.

[moio]

cc @MbolotSuse @CamrynCarter

[CamrynCarter]

This was already merged, but FWIW, looks good to me. Thanks @moio !