R1:43a6f34e7f2b

Update the SMTP (PHPMailer) adapter for the new mail API; remove "encoding" and "mailer"

Summary:
Ref T920. Ref T12404.

- Update to the new "$message" API.
- Remove "encoding". I believe "base64" is always the best value for this since we stopped seeing issues once we changed the default.
- Remove "mailer". This is a legacy option that makes little sense given how configuration now works.
- Rename to "SMTP". This doesn't affect users anymore since this mailer has been configured as `smtp` for…