External Libraries: Upgrade PHPMailer to version 6.6.4.
This is a maintenance release with minor changes:
* Add `text/csv` MIME type.
* Fix DKIM when sending to anonymous group via `mail()`.
Release notes:
https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.4
For a full list of changes in this update, see the PHPMailer GitHub:
https://github.com/PHPMailer/PHPMailer/compare/v6.6.3...v6.6.4
Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535].
Props jrf, Synchro.
Fixes #56016.
Built from https://develop.svn.wordpress.org/trunk@53917
git-svn-id: http://core.svn.wordpress.org/trunk@53476 1a063a9b-81f0-0310-95a4-ce76da25c4cd
eb822d7fef