Upgrade PHPMailer from 5.2.14 to 5.2.21.

The full list of changes is available here:
https://github.com/PHPMailer/PHPMailer/compare/v5.2.14...v5.2.21

Props sebastian.pisula, MattyRob, sfpt, dd32.
Fixes #37210 for trunk.

Built from https://develop.svn.wordpress.org/trunk@39645


git-svn-id: https://core.svn.wordpress.org/trunk@39585 1a063a9b-81f0-0310-95a4-ce76da25c4cd