your mail.php on config you declare host as smtp.mailgun.org and port is 587 while on env is different. you need to change your mail.php to

'host' => env('MAIL_HOST', 'mailtrap.io'),

'port' => env('MAIL_PORT', 2525),

if you desire to use mailtrap.Then run

php artisan config:cache

Swiftmailer 6.0 does not support the mail driver anymore

With laravel, you also updated swiftmailer, and thus, the mail driver does not work anymore.

Here is the swiftmailer commit that removed the mail driver in 6.0.

It was deprecated since Swiftmailer 5.4.5 with this commit.

Here is a long discussion about why it was removed, where the most useful comment says that:

PHP's mail() function is insecure

The fifth parameter can be exploited to execute arbitrary code on most Linux systems. This is mainly because PHP incorrectly escapes shell arguments. You can read the full explanation in "Why mail() is dangerous in PHP" by RIPS.

A couple of applications had security issues because of that, including Roundcube webmailer, MediaWiki, PHPMailer, ZendFramework, SquirrelMail and Swiftmailer. And your app. So be thankful that it got removed and use SMTP instead. You can usually simply use your local mail server using:

MAIL_DRIVER=smtp
MAIL_HOST=localhost
MAIL_PORT=25
MAIL_USERNAME=
MAIL_PASSWORD=
MAIL_ENCRYPTION=null

(You may need to use tls encryption if your server does not accept local unencrypted mail.)

Laravel

The main issue with Laravel is that its documentation still mentions the mail driver in the Laravel 5.7 documentation and did only mention in their upgrade notes for 5.5 that swiftmailer 6.0 is required, but not that this means you cannot use the mail driver anymore.