disallow unique directive on relationshipProperties explicitly, since interface rule is gone

Author: a-alle

packages/graphql/src/schema/validation/custom-rules/features/valid-relationship-properties.ts

@@ -79,6 +79,7 @@ function assertRelationshipProperties(
                 "subscriptionsAuthorization",
                 "relationship",
                 "cypher",
+                "unique",
             ];
             const foundForbiddenDirective = field.directives.find((d) => forbiddenDirectives.includes(d.name.value));
             if (foundForbiddenDirective) {