Mail: PHPMailer swap to use is_email for the default validator.

Prior to the PHPMailer update in 5.5, old version of the PHPMailer was setting the validator to 'auto' resulting in a sophisticated logic for determining what email address validation should be used. But the new version defaults to 'php', possibly leading to rejection of email addresses which were fine prior to the upgrade. Let's use the WordPress core function `is_email()` so that it can be fully pluggable.

Fixes #50720.
Props david.binda, ayeshrajans, Synchro, SergeyBiryukov, whyisjake.

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