PHPMailer: Update to v6.9.1

[jrf]

PHPMailer 6.9.1 has just been released. This release (and the unreleased v 6.9.0) a maintenance and feature release.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.9.1
• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.9.0
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.8.1...v6.9.1

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, #59238

[SergeyBiryukov]

In 57137:

External Libraries: Upgrade PHPMailer to version 6.9.1.

This is a maintenance and feature release, adding support for the official release of PHP 8.3, methods for removing and replacing custom headers, XCLIENT support, and links to a new way of implementing XOAUTH2 authentication.

The only change likely to have any impact on existing code is that PHPMailer previously attempted to use opportunistic STARTTLS encryption when connecting to localhost, which was unlikely to work. The workaround required setting SMTPAutoTLS = false, but that's no longer required. You may still need to use this setting when connecting to literal IPs.

References:

• ​PHPMailer 6.9.1 release notes
• ​Full list of changes in PHPMailer 6.9.1

Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535], [53917], [54427], [54937], [55557], [56484].

Props jrf, Synchro.
Fixes #59966.