Docs: update Airflow 3 auth-manager docs that still refer to removed webserver-era commands

[Dotify71]

Description

Some Airflow 3 auth-manager docs still appear to use pre-3 webserver-era wording, even though airflow webserver was removed and replaced by airflow api-server.

While checking the docs, I noticed this may affect pages around:

• providers/fab/docs/auth-manager/configuring-flask-app.rst
• providers/fab/docs/auth-manager/webserver-authentication.rst

and possibly a few nearby auth-manager pages that were carried forward from the older layout.

Why this is confusing

For someone following the Airflow 3 docs, references to the old command/runtime model make it harder to tell which setup steps are still valid.

The Airflow 3 release notes already state that airflow api-server replaced airflow webserver, so the surrounding auth-manager documentation should be consistent with that change.

Proposed update

A docs pass in this area should:

• replace outdated webserver command references with api-server where applicable
• review nearby auth-manager docs for similar Airflow 2 / pre-3 wording
• make sure examples and setup instructions match the current Airflow 3 CLI

Notes

I found this while looking for a small documentation issue to work on. If this direction makes sense, I can follow up with a PR to narrow the exact file/line changes.

[boring-cyborg]

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