External Libraries: Upgrade PHPMailer from 6.3.0 to 6.4.0.
6.4.0 reverts a change that made the `mail()` and sendmail transports set the envelope sender if one isn't explicitly provided, as it was causing problems in specific PHP/server configurations.
Release post: https://github.com/PHPMailer/PHPMailer/releases/tag/v6.4.0
Changelog: https://github.com/PHPMailer/PHPMailer/compare/v6.3.0...v6.4.0
Props Synchro, tigertech, ayeshrajans, galbaras, audrasjb, SergeyBiryukov, desrosj, ocean90.
Fixes #52822.
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