GHSA-wxwm-3fxv-mrvx: Directus: GraphQL Schema SDL Disclosure Setting

Directus had a security gap where turning off GraphQL introspection (a setting that hides database schema details from users) didn't actually work completely. Even though standard introspection queries were blocked, a different endpoint called `/graphql/system` still returned the same schema information in SDL format (schema definition language, a text representation of a database structure), allowing unauthenticated users to see what data collections and fields existed in the system.