Update PHPMailer to 6.6.4

[jrf]

PHPMailer 6.6.4 has just been released. It's only a small release with minimal changes for WP.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.4
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.6.3...v6.6.4

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

[SergeyBiryukov]

Thanks! Fixed in [53917], but I accidentally linked to the previous ticket in the commit :)

[jrf]

Happens to the best of us ;-) Thanks @SergeyBiryukov for creating the patch & getting this committed.