Before this patch, granting a user MODIFY permissions on ALL KEYSPACES allowed the user to write to system tables, where the user could also set himself to "superuser" granting him all other permissions. After this patch, MODIFY permissions on ALL KEYSPACES is limited only to non-system keyspaces.
Fixes: scylladb/scylladb#23218Closesscylladb/scylladb#23219