MDL-77350 completion: Added class properties that are not declared

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

completion/completion_completion.php

@@ -66,6 +66,8 @@ class completion_completion extends data_object {
     /* @var int Flag to trigger cron aggregation (timestamp) */
     public $reaggregate;
 
+    /** @var float user's course grade. */
+    public $gradefinal;
 
     /**
      * Finds and returns a data_object instance based on params.