add deprecation notice for SubDagOperator

[eladkal]

Adding deprecation notice for SubDagOperator
closes: #12292
The mailing list discussion was before Airflow 2 was released. I think we have a good adoption of TasksGroup however
I see users that are on Airflow 2 who still write new DAGs with SubDagOperator (Not sure why)

We know the general direction is to replace SubDagOperator with TasksGroup so I think we should place a deprecation notice.

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[potiuk]

I think Task Group does not (yet) provide full replacement to SubDag, but maybe that's good to deprecate it now to get all the uses from users.

[eladkal]

I think Task Group does not (yet) provide full replacement to SubDag, but maybe that's good to deprecate it now to get all the uses from users.

Yes this is what I had in mind.
We don't have to remove SubDagOperator in airflow 3 but we do want people to know that the feature is deprecated.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.