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.
Built from https://develop.svn.wordpress.org/trunk@44406


git-svn-id: http://core.svn.wordpress.org/trunk@44236 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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
  *

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44403';
+$wp_version = '5.1-alpha-44406';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.