Handle Drupal 9 deprecations

Any reason for not using Drupal\Component\EventDispatcher\Event in ViewsBulkOperationsActionManager as done in ViewsBulkOperationsEvent?

I'll review this.

After the #3, I still found some deprecated Event() warnings.

 ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------ 
  Line   modules/actions_permissions/src/EventSubscriber/ActionsPermissionsEventSubscriber.php                                                                       
 ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------ 
  37     Parameter $event of method Drupal\actions_permissions\EventSubscriber\ActionsPermissionsEventSubscriber::alterActions() has typehint with deprecated class  
         Symfony\Component\EventDispatcher\Event:                                                                                                                    
         since Symfony 4.3, use "Symfony\Contracts\EventDispatcher\Event" instead                                                                                    
 ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------ 

 ------ ------------------------------------------------------------------------------------------------------------------------------- 
  Line   src/ViewsBulkOperationsEvent.php                                                                                               
 ------ ------------------------------------------------------------------------------------------------------------------------------- 
  11     Class Drupal\views_bulk_operations\ViewsBulkOperationsEvent extends deprecated class Symfony\Component\EventDispatcher\Event:  
         since Symfony 4.3, use "Symfony\Contracts\EventDispatcher\Event" instead                                                       
 ------ ------------------------------------------------------------------------------------------------------------------------------- 

So I made a patch to kill all the Symfony\Component\EventDispatcher\Event is deprecated warnings. Kindly review it.

@heddn, actually there is a reason.. Drupal 8 doesn't have that class. Probably I should drop D8 support from now as it's becoming troublesome.

This is actually bigger as there are many other deprecations in automated tests. I'll handle it.

One unused use statement left, no point re-adding a patch. I'll commit without it.

Thanks!