General: Update since annotation for wp_unique_id().

In [43658], `wp_unique_id()` was introduced. This updates the `since` annotation to be accurate.

See #44883.

git-svn-id: https://develop.svn.wordpress.org/trunk@44406 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6209,7 +6209,7 @@ function wp_is_uuid( $uuid, $version = null ) {
  * with the optional prefix. As such the returned value is not universally unique,
  * but it is unique across the life of the PHP process.
  *
- * @since 4.9.9
+ * @since 5.0.3
  *
  * @staticvar int $id_counter
  *