Now that WordPress Core supports PHP >= 5.6, the PHPMailer library can be updated to the latest version.
The PHPMailer files now reside in a new directory, `wp-includes/PHPMailer`. These files are copied verbatim from the library upstream and will make updating in the future easier. For backwards compatibility, the old files will remain and trigger deprecated file warnings.
The PHPMailer class is also now under the `PHPMailer\PHPMailer\PHPMailer` namespace. The `PHPMailer` class in the global namespace has been aliased for a seamless transition.
This upgrade also clears up a handful of PHP compatibility issues detailed in #49922.
For a full list of changes, see the PHPMailer GitHub: https://github.com/PHPMailer/PHPMailer/compare/v5.2.27...v6.1.6.
Props Synchro, SergeyBiryukov, desrosj, donmhico, ayeshrajans.
Fixes#41750.
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