Use JSONB type for XCom.value column in PostgreSQL (apache#44290)

`JSONB` is more efficient, we already use that in migration, this makes it consistent when DB tables are created from ORM.

airflow/models/xcom.py

@@ -34,6 +34,7 @@
     select,
     text,
 )
+from sqlalchemy.dialects import postgresql
 from sqlalchemy.ext.associationproxy import association_proxy
 from sqlalchemy.orm import Query, reconstructor, relationship
 
@@ -79,7 +80,7 @@ class BaseXCom(TaskInstanceDependencies, LoggingMixin):
     dag_id = Column(String(ID_LEN, **COLLATION_ARGS), nullable=False)
     run_id = Column(String(ID_LEN, **COLLATION_ARGS), nullable=False)
 
-    value = Column(JSON)
+    value = Column(JSON().with_variant(postgresql.JSONB, "postgresql"))
     timestamp = Column(UtcDateTime, default=timezone.utcnow, nullable=False)
 
     __table_args__ = (