[core-api][experimental] allow_nonexistent_upstream_partitions

Author: OwenKephart

Diff for: python_modules/dagster/dagster/_core/definitions/time_window_partition_mapping.py

@@ -3,7 +3,7 @@
 from typing import NamedTuple, Optional, cast
 
 import dagster._check as check
-from dagster._annotations import PublicAttr, experimental_param
+from dagster._annotations import PublicAttr, beta_param
 from dagster._core.definitions.partition import (
     AllPartitionsSubset,
     PartitionsDefinition,
@@ -22,7 +22,7 @@
 
 
 @whitelist_for_serdes
-@experimental_param(param="allow_nonexistent_upstream_partitions")
+@beta_param(param="allow_nonexistent_upstream_partitions")
 class TimeWindowPartitionMapping(
     PartitionMapping,
     NamedTuple(