Remove global variables in airflow.settings

[jscheffl]

Another small increment to remove global statements for PR #58116 - an alternative approach to #61356.

This removes global statements from settings.py where the a lot of state was hold in variables used with global and then a lot of code way relying to access these globals. Is a like a public interface and refactoring would be needed.

global is evil.

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
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