External Libraries: Upgrade PHPMailer to version 6.8.1.
This is a minor maintenance release.
Note: WordPress core does not include PHPMailer's DSN class, as it is not particularly relevant for WP.
References:
* [https://github.com/PHPMailer/PHPMailer/releases/tag/v6.8.1 PHPMailer 6.8.1 release notes]
* [https://github.com/PHPMailer/PHPMailer/compare/v6.8.0...v6.8.1 Full list of changes in PHPMailer 6.8.1]
Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535], [53917], [54427], [54937], [55557].
Props jrf, Synchro.
Fixes #59238.
Built from https://develop.svn.wordpress.org/trunk@56484
git-svn-id: http://core.svn.wordpress.org/trunk@55996 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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