Examples/mailing_list: remove use of `E_STRICT`

While this is only example code, it is still code which should be functional for all supported PHP versions.

As `E_STRICT` has been deprecated since PHP 8.4 and wasn't really all that useful since PHP 7.0 anyway, it's probably better not to mention it in the example code.

Ref: https://wiki.php.net/rfc/deprecations_php_8_4#remove_e_strict_error_level_and_deprecate_e_strict_constant

examples/mailing_list.phps

@@ -8,7 +8,7 @@
 use PHPMailer\PHPMailer\PHPMailer;
 use PHPMailer\PHPMailer\Exception;
 
-error_reporting(E_STRICT | E_ALL);
+error_reporting(E_ALL);
 
 date_default_timezone_set('Etc/UTC');