PHPMailer: Remove use of each() function removed in PHP 8.0

Summary:
each() has been deprecated since PHP 7.2 and got removed in PHP 8.0 per https://www.php.net/manual/en/function.each.php

Replace it with a foreach construction.

Closes T15833

Test Plan: Carefully read the code; locally testing that construction per D25659#18155; compare to similar rP9623e667458821f696eb92e9dac5a2c9720af7af

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15833

Differential Revision: https://we.phorge.it/D25659