Upgrading Folio to work with Laravel 9

Laravel 9 was released on February 8, 2022. I've started adding official support for 9.x to Folio. (See the pull request.)

I've mostly added support for PHP ^8.0 and dropped support for PHP 7; updated many dependency packages—I had to edit the CommonMark implementation, which is slightly different in 2.x; and followed the upgrade guide which only affected the file system adapters.

Here are a few other things I need to take a look at.

• The mailing functionality broke as SwiftMailer is being removed in favor of Symfony Mailer. I need to check if this breaks Folio's notifications.
• The lang directory location has moved.

Note that I had to update the app/Http/Middleware/TrustProxies.php file in my apps, but this is not part of Folio. (I guess it could be beneficial to ship this via Folio to all apps.)