PHPMailer: Update to v6.8.1

[jrf]

PHPMailer 6.8.1 has just been released. This release contains a (very) minor security fix and some general maintenance fixes.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.8.1
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.8.0...v6.8.1

I've reviewed the diff and the changes look sensible to me, so I would recommend an update.

Previously: #53430, #53953, #54514, #55187, #55277, #55976, #56016, #56413, #56772, #57281, #57873

[SergeyBiryukov]

In 56484:

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:

• ​PHPMailer 6.8.1 release notes
• ​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.