Add dag_version cleanup indexes for task_instance and dag_run

[shaealh]

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Closes #60145

Adds a migration to speed up airflow db clean -t dag_version by creating missing indexes on:

• task_instance(dag_version_id) (idx_task_instance_dag_version_id)
• dag_run(created_dag_version_id) (idx_dag_run_created_dag_version_id)

Downgrade removes both indexes (if_exists=True).

Tests :

• ruff check airflow-core/src/airflow/migrations/versions/0108_3_2_0_add_dag_version_id_indexes_for_db_cleanup.py
• pytest airflow-core/tests/unit/utils/test_db.py -k single_head_revision_in_migrations -q --with-db-init
• pytest airflow-core/tests/unit/migrations/test_no_orm_refs_in_migration_scripts.py -q

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[shaealh]

Please review