Fix PHPStan

Problem/Motivation

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  Line   modules/custom/devel_php/src/Form/ExecutePHP.php                                                              
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  33     Method Drupal\devel_php\Form\ExecutePHP::create() has no return typehint specified.                           
  52     Method Drupal\devel_php\Form\ExecutePHP::buildForm() has parameter $details_open with no typehint specified.  
  90     Method Drupal\devel_php\Form\ExecutePHP::submitForm() has no return typehint specified.                       
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  Line   modules/custom/devel_php/src/Plugin/Block/DevelExecutePHP.php                                   
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  57     Unsafe usage of new static().                                                                   
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static                
  76     Method Drupal\Core\Form\FormBuilderInterface::getForm() invoked with 2 parameters, 1 required.  
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