External Libraries: Add class aliases for `phpmailerException` and `SMTP` to...

External Libraries: Add class aliases for `phpmailerException` and `SMTP` to deprecated files to account for the new namespace.

This ensures backward compatibility with plugins using the old `class-phpmailer.php` or `class-smtp.php` files.

Follow-up to [48033].

Props Otto42.
Fixes #50379. See #41750.
Built from https://develop.svn.wordpress.org/trunk@48035


git-svn-id: https://core.svn.wordpress.org/trunk@47802 1a063a9b-81f0-0310-95a4-ce76da25c4cd