Clarify warning for partition-driven DAGs in dags next-execution

[shleshaG]

What does this PR do?

Clarifies the warning message shown by airflow dags next-execution when no next
data interval can be determined, noting that this can also occur for
dataset- or partition-driven DAGs.

Why is this needed?

With AIP-76, partition-driven and dataset-driven DAGs may not have a
determinable next execution date. The previous message only mentioned
@once or None schedules, which could be misleading.

This change improves CLI clarity without altering behavior.

How was this tested?

• Manual verification of CLI output

Related issue

• Determine what next execution should do for partition-driven dags #61076

[boring-cyborg]

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