Required parameters $password and $username follow the optional parameter $tval.
This is deprecated since PHP 8.0 per https://www.php.net/manual/en/migration80.deprecated.php
Define defaults for those required parameters to fix the issue - https://github.com/PHPMailer/PHPMailer/blob/master/src/POP3.php#L199 seems to also have survived so far without complaints by PHP 8 users (and PHPStan also does not complain anymore after this change).
Closes T15834