External Libraries: Update the SimplePie library to version 1.5.6.

This version fixes a handful of PHP 8 compatibility issues.

A full list of changes included in this update can be found on GitHub: https://github.com/simplepie/simplepie/compare/1.5.5...1.5.6.

Props jrf, ayeshrajans.
Fixes #51521.

git-svn-id: https://develop.svn.wordpress.org/trunk@49176 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-simplepie.php

@@ -68,7 +68,7 @@ spl_autoload_register( 'wp_simplepie_autoload' );
  * POSSIBILITY OF SUCH DAMAGE.
  *
  * @package SimplePie
- * @version 1.5.5
+ * @version 1.5.6
  * @copyright 2004-2017 Ryan Parman, Sam Sneddon, Ryan McCue
  * @author Ryan Parman
  * @author Sam Sneddon
@@ -85,7 +85,7 @@ define('SIMPLEPIE_NAME', 'SimplePie');
 /**
  * SimplePie Version
  */
-define('SIMPLEPIE_VERSION', '1.5.5');
+define('SIMPLEPIE_VERSION', '1.5.6');
 
 /**
  * SimplePie Build