PHPMailer: Update to v6.10.0

[agulbra]

PHPMailer 6.10.0 was released a few months ago. This release is a maintenance and feature release.

Full details:

​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.10.0
​https://github.com/PHPMailer/PHPMailer/compare/v6.9.1...v6.10.0

Trac ticket: https://core.trac.wordpress.org/ticket/63811

[SergeyBiryukov]

In 60623:

External Libraries: Upgrade PHPMailer to version 6.10.0.

This is a feature and maintenance release introducing full support for ​RFC 6531 SMTPUTF8, meaning that plugin or theme developers are now free to use Unicode characters in email addresses, such as JøeÜser@example.com, without any complicated encoding schemes. Using this feature requires sending through a mail server that advertises support for SMTPUTF8. For full details see ​SMTPUTF8.md.

This commit also includes the parts of PHPMailer not previously bundled with core, specifically the DSNConfigurator, OAuth, and POP3 classes, so that plugin developers could use those extended features without including their own versions of the library.

Including the full library aims to make it easier (and faster) for core to update in case of security issues, and to provide more flexibility and security for plugins and (by extension) users of WordPress.

References:

• ​PHPMailer 6.10.0 release notes
• ​Full list of changes in PHPMailer 6.10.0

Follow-up to [54937], [55557], [56484], [57137], [59246], [59481].

Props agulbra, Ipstenu, JeffMatson, lukecavanagh, dd32, Otto42, JeffMatson, MattyRob, desrosj, SirLouen, SergeyBiryukov.
Fixes #39714, #63811.