Update PHPMailer to 6.5.3

[jrf]

PHPMailer 6.5.3 has just been released and includes official PHP 8.1 support, as well as some small bugfixes, including one for PHP 5.6 cross-version support.

The version included with WP should be updated.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.2
• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.3
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.5.1...v6.5.3

I've reviewed the diff and the changes look sensible to me, so I would recommend an update at the earliest convenience.

Previously: #53430, #53953

[SergeyBiryukov]

In 52252:

External Libraries: Upgrade PHPMailer to version 6.5.3.

The latest release includes official PHP 8.1 support, as well as some small bug fixes, including one for PHP 5.6 cross-version support.

Release notes:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.2
• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.3

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.5.1...v6.5.3

Follow-up to [50628], [50799], [51169], [51634], [51635].

Props jrf, Synchro.
Fixes #54514.