Update PHPMailer to 6.3.0

[ayeshrajans]

​PHPMailer 6.3.0 is released with bug fixes.

[ayeshrajans]

Updating with verbatim v6.3.0 from Github, and patching with 3 hunks for PHPCS rules.

[SergeyBiryukov]

In 50397:

External Libraries: Upgrade PHPMailer to version 6.3.0.

This is a maintenance release. Changes include:

• Handle early connection errors such as 421 during connection and EHLO states.
• Make the mail() and sendmail transports set the envelope sender the same way as SMTP does, i.e. use whatever From is set to, only falling back to the sendmail_from php.ini setting if From is unset. This avoids errors from the mail() function if Sender is not set explicitly and php.ini is not configured. This is a minor functionality change, so bumps the minor version number.
• Extend parseAddresses to decode encoded names.

Release notes: ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.3.0

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.2.0...v6.3.0

Props ayeshrajans.
Fixes #52577.

[desrosj]

This was mentioned in the miscellaneous developer changes developer note: ​https://make.wordpress.org/core/2021/02/23/miscellaneous-developer-focused-changes-in-wordpress-5-7/.