Query: Add since annotation to WP_Query::$query_cache_key.

Follow up to [59766].

Props mukesh27.
See #59516.


git-svn-id: https://develop.svn.wordpress.org/trunk@59771 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-query.php

@@ -480,6 +480,7 @@ class WP_Query {
 	 * The cache key is generated by the method ::generate_cache_key() after the
 	 * query has been normalized.
 	 *
+	 * @since 6.8.0
 	 * @var string
 	 */
 	private $query_cache_key = '';