[9.x] Update from Swift Mailer to Symfony Mailer

[Jubeki]

Laravel 9.x will use the Symfony Mailer instead of the discontinued SwiftMailer.
Waiting for laravel/framework#38481 to be merged.

Changes:

• Guzzle is no longer needed because symfony-http will be used by the API Transports instead.
• Each Transport now has now a symfony transport install (except array and log)

Unchanged:

• Config entries

Some things which should be discussed:

• Laravel 9.x will use Symfony 6.0 therefor the Links are absolut to https://symfony.com/doc/6.0/mailer.html there also exists a link to the current documentation if that is wished for: https://symfony.com/doc/current/mailer.html (Currently v5.3)
• Symfony Mailer is based on Symfony Mime which could be mentioned somewhere if necessary.

[driesvints]

Thank you @Jubeki