Update PHPMailer to 6.6.3

[jrf]

PHPMailer 6.6.3 has just been released. It's only a small release with minimal changes for WP.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.3
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.6.2...v6.6.3

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

Previously: #53430, #53953, #54514, #55187, #55277, #55976

[SergeyBiryukov]

In 53535:

External Libraries: Upgrade PHPMailer to version 6.6.3.

This is a maintenance release with minor CS improvements.

Release notes:
​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.3

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.6.2...v6.6.3

Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500].

Props jrf, Synchro.
Fixes #56016.

[SergeyBiryukov]

In 53917:

External Libraries: Upgrade PHPMailer to version 6.6.4.

This is a maintenance release with minor changes:

• Add text/csv MIME type.
• Fix DKIM when sending to anonymous group via mail().

Release notes:
​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.6.4

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.6.3...v6.6.4

Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535].

Props jrf, Synchro.
Fixes #56016.

[SergeyBiryukov]

For reference, #56413 was the correct ticket for [53917].