PHPMailer: Set defaults for required parameters followed by optional parameters in Authorise() method signature

Summary:
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

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15834

Differential Revision: https://we.phorge.it/D25660