Fix PHP 8.5 __sleep deprecation (#1997)

This patch breaks any implementation of __sleep in child classes/user handlers. Should be warned in the release notes.

Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>

tests/Monolog/Handler/StreamHandlerTest.php

@@ -67,7 +67,7 @@ class StreamHandlerTest extends \Monolog\Test\MonologTestCase
 
     /**
      * @covers Monolog\Handler\StreamHandler::close
-     * @covers Monolog\Handler\Handler::__sleep
+     * @covers Monolog\Handler\Handler::__serialize
      */
     public function testSerialization()
     {