Improve precision of the remove_task_decorator function

[SamWheating]

closes: #34154

The remove_task_decorator can incorrectly truncate function bodies if there are specific comments in the function body present, which is a rare edge case but can lead to really misleading behaviour where only a part of the intended code is executed.

I think that the correct way to do code manipulation like this would be to lean on the ast library, but the ability to go from ast back to code was only added in python 3.9 with the ast.unparse() function.

Anyways, this PR is an attempt to improve the accuracy of this method. While I don't think that this is perfect, it at least solves the issue for the cases in the linked issue with the following changes:

• only early exiting if a line starts with the decorator name
• splitting the code at most one time

Any suggestions welcome.

If you want to discuss this IRL, come find me at Airflow Summit :)

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