[airflow] Add warning to airflow.datasets.DatasetEvent usage (AIR301) (#20551)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

`airflow.datasets.DatasetEvent` has been removed in 3 but `AssetEvent`
might be added in the future

## Test Plan

<!-- How was it tested? -->

update the test fixture and reorg in the second commit

Author: Lee-W

crates/ruff_linter/resources/test/fixtures/airflow/AIR301_names.py

@@ -11,7 +11,7 @@
 )
 from airflow.api_connexion.security import requires_access
 from airflow.contrib.aws_athena_hook import AWSAthenaHook
-from airflow.datasets import DatasetAliasEvent
+from airflow.datasets import DatasetAliasEvent, DatasetEvent
 from airflow.operators.postgres_operator import Mapping
 from airflow.operators.subdag import SubDagOperator
 from airflow.secrets.cache import SecretCache
@@ -48,6 +48,7 @@
 
 # airflow.datasets
 DatasetAliasEvent()
+DatasetEvent()
 
 
 # airflow.operators.subdag.*

crates/ruff_linter/src/rules/airflow/rules/removal_in_3.rs

@@ -655,6 +655,11 @@ fn check_name(checker: &Checker, expr: &Expr, range: TextRange) {
         },
         // airflow.datasets
         ["airflow", "datasets", "DatasetAliasEvent"] => Replacement::None,
+        ["airflow", "datasets", "DatasetEvent"] => Replacement::Message(
+            "`DatasetEvent` has been made private in Airflow 3. \
+             Use `dict[str, Any]` for the time being. \
+             An `AssetEvent` type will be added to the apache-airflow-task-sdk in a future version.",
+        ),
 
         // airflow.hooks
         ["airflow", "hooks", "base_hook", "BaseHook"] => Replacement::Rename {