Mail: changed default "MAIL_FROM" address

Used an "example.com" address so we're using a propoer reserved domain,
and to avoid these trying to be delivered to the main bookstackapp
domain.

Closes #4518

.env.example.complete

@@ -72,7 +72,7 @@ MYSQL_ATTR_SSL_CA="/path/to/ca.pem"
 # Mail configuration
 # Refer to https://www.bookstackapp.com/docs/admin/email-webhooks/#email-configuration
 MAIL_DRIVER=smtp
-MAIL_FROM=mail@bookstackapp.com
+MAIL_FROM=bookstack@example.com
 MAIL_FROM_NAME=BookStack
 
 MAIL_HOST=localhost

app/Config/mail.php

@@ -22,7 +22,7 @@ return [
 
     // Global "From" address & name
     'from' => [
-        'address' => env('MAIL_FROM', 'mail@bookstackapp.com'),
+        'address' => env('MAIL_FROM', 'bookstack@example.com'),
         'name'    => env('MAIL_FROM_NAME', 'BookStack'),
     ],