Update PHPMailer to 6.5.4

[jrf]

PHPMailer 6.5.4 has just been released. It includes some minor PHP cross-version improvements and a safeguard against hosters disabling security functions.

The version included with WP should be updated.

Full details:

• ​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.4
• ​https://github.com/PHPMailer/PHPMailer/compare/v6.5.3...v6.5.4

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, #54514

[SergeyBiryukov]

In 52749:

External Libraries: Upgrade PHPMailer to version 6.5.4.

The latest release includes some minor PHP cross-version improvements and a safeguard against hosters disabling security functions. Note to hosting providers: don't disable escapeshellarg() and escapeshellcmd(); it's not safe!

Release notes:
​https://github.com/PHPMailer/PHPMailer/releases/tag/v6.5.4

For a full list of changes in this update, see the PHPMailer GitHub:
​https://github.com/PHPMailer/PHPMailer/compare/v6.5.3...v6.5.4

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

Props jrf, Synchro.
Fixes #55187.

[milana_cap]

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