Docs: update FAB auth manager Flask config wording for Airflow 3

[Dotify71]

Summary

Updates the FAB auth manager Flask configuration docs to use current Airflow 3 terminology.

Specifically, this change:

• renames the page title from "Airflow Webserver" to "Airflow UI"
• removes the outdated reference to "Airflow 2 plugins"
• clarifies that webserver_config.py is loaded by the FAB auth manager in this setup

Why

Airflow 3 moved away from older webserver-era wording in several places, but this page still used terminology that reads like pre-3 documentation. Tightening the wording here should make the page less confusing for users following the current auth-manager docs.

Closes #68159.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example Dag that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack