More proper default value for namespace in K8S cleanup-pods CLI (#15060)

Currently the default value for namespace is always 'default'.

However, `conf.get('kubernetes', 'namespace')` may be a more proper
default value for namespace in this case

(cherry picked from commit b8cf46a)

airflow/cli/cli_parser.py

@@ -678,8 +678,8 @@ def positive_int(value):
 # kubernetes cleanup-pods
 ARG_NAMESPACE = Arg(
     ("--namespace",),
-    default='default',
-    help="Kubernetes Namespace",
+    default=conf.get('kubernetes', 'namespace'),
+    help="Kubernetes Namespace. Default value is `[kubernetes] namespace` in configuration.",
 )
 
 ALTERNATIVE_CONN_SPECS_ARGS = [