Remove timeout from airflow.utils and update core usages accordingly

[amoghrajesh]

Part of #51671

Refactor timeout functionality by moving TimeoutPosix class to the dagbag module where it's primarily used, and remove windows timeout support along the way as Airflow 3.0 no longer supports windows due to fork() usage.

Since this is the final set of usage in core, providers is handled by: #54183 and the class was moved to task SDK in #54089. I am deprecating this from utils and asking dag authors to use it from sdk.

Testing

• Updated existing timeout tests to use new import path
• Verified DAG import timeout functionality still works
• Confirmed backward compatibility with timeout alias

Back compat:

root@8f4a0f87f9f0:/opt/airflow# python
Python 3.10.18 (tags/v3.10.18:88663ef, Jul 22 2025, 04:10:51) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from airflow.utils.timeout import timeout
/opt/airflow/airflow-core/src/airflow/configuration.py:814 DeprecationWarning: The grid_view_sorting_order option in [webserver] has been moved to the grid_view_sorting_order option in [api] - the old setting has been used, but please update your config.
/opt/airflow/airflow-core/src/airflow/configuration.py:815 DeprecationWarning: The grid_view_sorting_order option in [webserver] has been moved to the grid_view_sorting_order option in [api] - the old setting has been used, but please update your config.
<stdin>:1 DeprecationWarning: The `airflow.utils.timeout.timeout` attribute is deprecated. Please use `'airflow.sdk.execution_time.timeout.timeout'`.

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