General: Add missing `@since` annotation for `wp_get_direct_php_update_url()`.

Missed in [44815].

Props birgire.
See #46074.

git-svn-id: https://develop.svn.wordpress.org/trunk@44829 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6825,6 +6825,8 @@ function wp_update_php_annotation() {
  * by using the {@see 'wp_direct_php_update_url'} filter. This allows hosts to send users directly to
  * the page where they can update PHP to a newer version.
  *
+ * @since 5.1.1
+ *
  * @return string URL for directly updating PHP or empty string.
  */
 function wp_get_direct_php_update_url() {