Update PHPMailer to 6.6.0

[jrf]

PHPMailer 6.6.0 has just been released.

From the release notes:

Prior to this version, any OAuth provider needed to extend the provided OAuth base class, and this made it difficult to use with libraries other than ones based on the default league client packages. > The OAuth property now accepts anything that implements the OAuthProviderInterface, making it much easier to use things like Google's own OAuth classes. Existing implementations that extend the provided OAuth base class will still work, as that base class now implements this interface too. Thanks to @pdscopes.

When TLS errors occurred in PHPMailer, the error messages were often missing important info that might help diagnose/solve the problem. These error messages should now be more informative. A minor change is that a TLS error on SMTP connect will now throw an exception if exceptions are enabled. Thanks to @miken32.

The version included with WP should be updated.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.0
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.5.4...v6.6.0

I've reviewed the diff and I don't see anything which concerns me, so I would recommend an update at the earliest convenience.

Previously: #53430, #53953, #54514, #55187

[SergeyBiryukov]

In 52811:

External Libraries: Upgrade PHPMailer to version 6.6.0.

This is a minor feature release.

Prior to this version, any OAuth provider needed to extend the provided OAuth base class, and this made it difficult to use with libraries other than ones based on the default ​league client packages. The OAuth property now accepts anything that implements the OAuthProviderInterface, making it much easier to use things like ​Google's own OAuth classes. Existing implementations that extend the provided OAuth base class will still work, as that base class now implements this interface too.

When TLS errors occurred in PHPMailer, the error messages were often missing important info that might help diagnose/solve the problem. These error messages should now be more informative. A minor change is that a TLS error on SMTP connect will now throw an exception if exceptions are enabled.

Release notes:
​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.0

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.5.4...v6.6.0

Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749].

Props jrf, Synchro, miken32.
Fixes #55277.

[milana_cap]

Marking for dev note but really needs only mentioning in Field guide.