Clarify note re: webserver_config.py deprecation

[bgshacklett]

This pull request aims to clarify the status and usage of webserver_config.py. It is currently noted to be deprecated, though discussions point to the contrary, and it does not currently appear possible to set the AUTH configuration key for FAB Auth Manager in any other way.

related: #53573, #59475

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.