streams: Update error message returned for administrative access.

We have now decided to not continue with the stream administrator
concept as we are changing the permissions model to be based on
user groups as per #19525. So, this commit updates the error message
to "Must be an organization administrator".

zerver/lib/streams.py

@@ -9,8 +9,8 @@ from django.utils.translation import gettext as _
 from zerver.actions.default_streams import get_default_streams_for_realm
 from zerver.lib.exceptions import (
     JsonableError,
+    OrganizationAdministratorRequired,
     OrganizationOwnerRequired,
-    StreamAdministratorRequired,
 )
 from zerver.lib.markdown import markdown_convert
 from zerver.lib.stream_subscription import (
@@ -315,7 +315,7 @@ def check_stream_access_for_delete_or_update(
     if sub is not None and sub.is_stream_admin:
         return
 
-    raise StreamAdministratorRequired()
+    raise OrganizationAdministratorRequired()
 
 
 def access_stream_for_delete_or_update(