Added missing return type to SMTPDotStuffingFilter::filter

[vortrixs]

Just triggered this error

During inheritance of php_user_filter: Uncaught ErrorException: Return type of Kodus\Mail\SMTP\SMTPDotStuffingFilter::filter($in, $out, &$consumed, $closing) should either be compatible with php_user_filter::filter($in, $out, &$consumed, bool $closing): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /var/www/projects/jourbox/vendor/kodus/mail/src/SMTP/SMTPDotStuffingFilter.php:38

No idea why the tests aren't failing since this filter is used every time a mail is send 😕