External Libraries: Upgrade PHPMailer to version 6.11.0.

This is a large maintenance release with address parser improvements.

References:
* [https://github.com/PHPMailer/PHPMailer/releases/tag/v6.11.0 PHPMailer 6.11.0 release notes]
* [PHPMailer/PHPMailer@v6.10.0...v6.11.0 Full list of changes in PHPMailer 6.11.0]

Follow-up to [54937], [55557], [56484], [57137], [59246], [59481], [60623].

Props SirLouen, jrf, mukesh27, SergeyBiryukov.
Fixes #64052.

git-svn-id: https://develop.svn.wordpress.org/trunk@60813 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov