Inherit AirflowNotFound from connextion.NotFound for better readability

[mattwang44]

These lines in post_set_task_instances_state() tends to

1. raise NotFound if no dag return based on the given dag_id
2. catch exception SerializedDagNotFound and raise NotFound

but raising exception in a try-catch clause lowers the readability.

This PR makes AirflowNotFoundException inherited from connextion.NotFound and thus the try-catch can be removed and make post_set_task_instances_state() simpler.

Fixes: #11851

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[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.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!