How do I move a site to a new server?
It seems that you may forgot to move the vendor directory to the new location. Also you may need to run composer dump-autoload to update the autoloader with your new vendor's path. Though may be obvious to many, you need to clear the Drupal cache, in this case I prefer do a truncate over the cache bins (assuming you're using the default cache backend).

Update

Also worth checking the autoload.php file in your Drupal root, this file includes the actual autoloader generated by Composer.