mail/phpmailer6: Update to 6.4.0

Changelog:

- Revert change that made the mail() and sendmail transports set the envelope sender if one isn't explicitly provided, as it causes problems described in https://github.com/PHPMailer/PHPMailer/issues/2298
- Check for mbstring extension before decoding addresss in parseAddress
- Add Serbian Latin translation (sr_latn)
- Enrol PHPMailer in Tidelift

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