Drupal 9 compatibility for Config Patch

There are some issues with the module on Drupal 9.

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  Line   src/EventSubscriber/ConfigSubscriber.php                               
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  51     Parameter $event of method                                             
         Drupal\config_patch\EventSubscriber\ConfigSubscriber::onConfigChange(  
         ) has typehint with deprecated class                                   
         Symfony\Component\EventDispatcher\Event:                               
         since Symfony 4.3, use "Symfony\Contracts\EventDispatcher\Event"       
         instead                                                                
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  Line   tests/src/Functional/LoadTest.php                                    
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  13     Drupal\Tests\BrowserTestBase::$defaultTheme is required. See         
         https://www.drupal.org/node/3083055, which includes recommendations  
         on which theme to use.                                               
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 [ERROR] Found 2 errors