Update PHPMailer to 6.8.0

[jrf]

PHPMailer 6.8.0. has just been released. Mostly a bug fix release, though it also contains a new feature (DSN URL parsing).

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.8.0
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.7...v6.8.0

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

To be decided: should the new src/DSNConfigurator.php file be included in WP or not ?

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

[jrf]

Duplicate of #57873.