Move aliases outside of the php version check

See https://github.com/cakephp/cakephp/pull/6324\#discussion_r28206164

config/bootstrap.php

@@ -19,14 +19,16 @@ define('TIME_START', microtime(true));
 
 // @deprecated Backward compatibility with 2.x series
 if (PHP_VERSION_ID < 70000) {
-    class_alias('Cake\Mailer\AbstractTransport', 'Cake\Network\Email\AbstractTransport');
-    class_alias('Cake\Mailer\Transport\DebugTransport', 'Cake\Network\Email\DebugTransport');
-    class_alias('Cake\Mailer\Email', 'Cake\Network\Email\Email');
-    class_alias('Cake\Mailer\Transport\MailTransport', 'Cake\Network\Email\MailTransport');
-    class_alias('Cake\Mailer\Transport\SmtpTransport', 'Cake\Network\Email\SmtpTransport');
     class_alias('Cake\Utility\Text', 'Cake\Utility\String');
 }
 
+// @deprecated Backward compatibility with 2.x, 3.0.x
+class_alias('Cake\Mailer\AbstractTransport', 'Cake\Network\Email\AbstractTransport');
+class_alias('Cake\Mailer\Transport\DebugTransport', 'Cake\Network\Email\DebugTransport');
+class_alias('Cake\Mailer\Email', 'Cake\Network\Email\Email');
+class_alias('Cake\Mailer\Transport\MailTransport', 'Cake\Network\Email\MailTransport');
+class_alias('Cake\Mailer\Transport\SmtpTransport', 'Cake\Network\Email\SmtpTransport');
+
 require CAKE . 'basics.php';
 
 // Sets the initial router state so future reloads work.