Refs #35444 -- Deprecated contrib.postgres.OrderableAggMixin

This commit does not create any functional changes, but marks the
existing `OrderableAggMixin` class as deprecated so that developers
using it directly can be made aware of its future removal.

django/contrib/postgres/aggregates/mixins.py

@@ -23,4 +23,10 @@ class OrderableAggMixin(_DeprecatedOrdering):
     allow_order_by = True
 
     def __init_subclass__(cls, /, *args, **kwargs):
+        warnings.warn(
+            "OrderableAggMixin is deprecated. Use Aggregate and allow_order_by "
+            "instead.",
+            category=RemovedInDjango61Warning,
+            stacklevel=1,
+        )
         super().__init_subclass__(*args, **kwargs)

docs/internals/deprecation.txt

@@ -28,6 +28,9 @@ details on these changes.
 
 * The ``django.contrib.postgres.aggregates.StringAgg`` class will be removed.
 
+* The ``django.contrib.postgres.aggregates.mixins.OrderableAggMixin`` class
+  will be removed.
+
 .. _deprecation-removed-in-6.0:
 
 6.0

docs/releases/5.2.txt

@@ -499,3 +499,6 @@ Miscellaneous
 
 * The PostgreSQL ``StringAgg`` class is deprecated in favor of the generally
   available ``StringAgg`` class.
+
+* The PostgreSQL ``OrderableAggMixin`` is deprecated in favor of the
+  ``order_by`` attribute now available on the ``Aggregate`` class.