Deprecate subprocess exec utils from airflow core

[amoghrajesh]

Building up on top of #57187 and #57189, there are no more usages of these utilities.

Adding a deprecation warning to warn users to move away from using these if they are using it. There are some related unit tests too, which I am not so sure if we should remove now or later because technically this bit of code still will work until we wipe it off the repo.

This is how the deprecation warning looks like:

execute_in_subprocess(cmd=["bash", "-c", "echo CAT; echo KITTY;"])
<ipython-input-3-0c1513dd948a>:1 DeprecatedImportWarning: The function `execute_in_subprocess` is deprecated and will be removed in a future version.
/Users/amoghdesai/Documents/OSS/repos/airflow/airflow-core/src/airflow/utils/process_utils.py:188 DeprecatedImportWarning: The function `execute_in_subprocess_with_kwargs` is deprecated and will be removed in a future version.
[2025-10-24T11:18:13.833774Z] {process_utils.py:209} INFO - Executing cmd: bash -c 'echo CAT; echo KITTY;'
[2025-10-24T11:18:13.844192Z] {process_utils.py:213} INFO - Output:
[2025-10-24T11:18:13.851401Z] {process_utils.py:217} INFO - CAT
[2025-10-24T11:18:13.851532Z] {process_utils.py:217} INFO - KITTY
execute_in_subprocess_with_kwargs(cmd=["bash", "-c", "echo CAT; echo KITTY;"])
<ipython-input-4-46ee57e90afd>:1 DeprecatedImportWarning: The function `execute_in_subprocess_with_kwargs` is deprecated and will be removed in a future version.
[2025-10-24T11:18:23.570528Z] {process_utils.py:209} INFO - Executing cmd: bash -c 'echo CAT; echo KITTY;'
[2025-10-24T11:18:23.578198Z] {process_utils.py:213} INFO - Output:
[2025-10-24T11:18:23.582314Z] {process_utils.py:217} INFO - CAT
[2025-10-24T11:18:23.582415Z] {process_utils.py:217} INFO - KITTY

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