External Libraries: Upgrade PHPMailer to version 6.7.
This is a bug fix release which also contains some PHP 8.1 related improvements.
References:
* [https://github.com/PHPMailer/PHPMailer/releases/tag/v6.7 PHPMailer 6.7 release notes]
* [https://github.com/PHPMailer/PHPMailer/compare/v6.6.5...v6.7 Full list of changes in PHPMailer 6.7]
Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535], [53917], [54427].
Props jrf, Synchro.
Fixes #57281.
Built from https://develop.svn.wordpress.org/trunk@54937
git-svn-id: http://core.svn.wordpress.org/trunk@54489 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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