[`airflow`] Improve the `AIR002` error message (#20173)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
### What
Change the message from "DAG should have an explicit `schedule`
argument" to "`DAG` or `@dag` should have an explicit `schedule`
argument"
### Why
We're trying to get rid of the idea that DAG in airflow was Directed
acyclic graph. Thus, change it to refer to the class `DAG` or the
decorator `@dag` might help a bit.
## Test Plan
<!-- How was it tested? -->
update the test fixtures accordly
