Add indexes on task_instance.dag_version_id and dag_run.created_dag_version_id

[justinpakzad]

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Added indexes on task_instance.dag_version_id and dag_run.created_dag_version_id. This should prevent sequential scans when running command such as airflow db clean -t dag_version --clean-before-timestamp 2025-12-31.

closes: #60145

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