Yii 2 Symfony mailer extension 4.0

Major version of Yii 2 Symfony mailer extension package was tagged.

In this version:

• Add option to create transport from DSN object.
• Add a security notice about the DSN.
• If yiisoft/yii2-psr-log-source is available, the extension now wirtes logs.
• Remove unnecessary dependency on the Symfony mailer class.
• Proxy transport events to events on the mailer component.
• Make message class inheritable again.
• Drop support for PHP < 8.1.
• Use the DI container for creating transport factories.