Update PHPMailer to 6.5.1

[jrf]

PHPMailer 6.5.1 has just been released and includes preliminary PHP 8.1 support, as well as some small bugfixes.

The version included with WP should be updated.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.1
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.5.0...v6.5.1 (note: this diff is massive, but most changes are in the test suite)

I've reviewed the (code, not test) diff and would strongly recommend an update at the earliest convenience. Second opinion appreciated though as I made a significant number of the code changes, so I'm biased. 😁

Previously: #53430
Related: #53635

[SergeyBiryukov]

In 51634:

External Libraries: Upgrade PHPMailer to version 6.5.1.

The latest release includes preliminary PHP 8.1 support, as well as some small bug fixes.

Release notes: ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.1

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.5.0...v6.5.1

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

Props jrf.
Fixes #53953.

[SergeyBiryukov]

In 51635:

External Libraries: Restore the phpcs:ignore statements in PHPMailer.

These are necessary for a passing PHP Compatibility scan.

Follow-up to [48045], [51634].

See #53953.

[jrf]

@SergeyBiryukov Cricky! You're fast with these external library updates 😁

FYI: once PHPCompatibility 10.0 has been released, PHPMailer will inline the relevant phpcs:ignore statements as well, so after that, the update should be even easier ;-)