Suppress notice for return type mixed

src/Entities/ScopeEntity.php

@@ -36,7 +36,8 @@ class ScopeEntity implements ScopeEntityInterface, CacheableDependencyInterface
   /**
    * {@inheritdoc}
    */
-  public function jsonSerialize(): mixed {
+  #[\ReturnTypeWillChange]
+  public function jsonSerialize() {
     return $this->getIdentifier();
   }