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

externals/phpmailer/class.pop3.php

@@ -142,7 +142,7 @@ class POP3 {
    * @param string $username
    * @param string $password
    */
-  public function Authorise ($host, $port = false, $tval = false, $username, $password, $debug_level = 0) {
+  public function Authorise ($host, $port = false, $tval = false, $username = '', $password = '', $debug_level = 0) {
     $this->host = $host;
 
     //  If no port value is passed, retrieve it