Considerations for PHPMailer v6 & v7

[petecooper]

https://github.com/PHPMailer/PHPMailer/releases
https://github.com/textpattern/textpattern/tree/dev/textpattern/vendors/phpmailer/phpmailer

We use PHPMailer in vendors and we avoided any fallout from PHPMailer/PHPMailer#3243 by doing…pretty much nothing. We have an opportunity to continue tracking PHPMailer v6 or switch to v7.

Looking at the project releases, there's currently more dev & release activity in the v7 branch, which could be positive for future changes that we could make use of. Switching to v7 would mean some work for us in the short term, but the benefits may make that worthwhile. I have limited visibility of what that work would look like, so I defer to @Bloke / @bloatware / @jools-r for guidance.

Considerations:

• Pin Textpattern 4.9 to PHPMailer 6.; use PHPMailer 7. for Textpattern 5.0. Some extra housekeeping to maintain a 'split'.
• Use PHPMailer 7.* for both, make the changes in the 4.9.1 or 4.9.2 release since they're under the patch umbrella (as I understand it).
• Use PHPMailer 6.* for both until we retire Textpattern 4.9 and then figure out the migration to PHPMailer current version in due course for whatever dev is on.