Show each Dag only once in airflow dags list

[Eason09053360]

Airflow 3 stores one serialized_dag row per Dag version, but airflow dags list
selected every row without filtering by version, so a Dag was printed once per
version it had.

Use the existing SerializedDagModel.read_all_dags() helper, which selects the
row with the highest version_number per dag_id.

Note that #60868 / #61077 fixed one source of spurious version growth, but
legitimate versions still accumulate whenever a Dag is edited, so the duplicate
rows remain reproducible.

closes: #60594

This supersedes #60637, which reached the same diagnosis but was closed by the
stale bot without review.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

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