Drupal 11 compatibility

Currently there are two issues in phpstan:

  Line   src/Form/UserDefaultPageConfigEntityForm.php                           
 ------ ----------------------------------------------------------------------- 
  103    Call to deprecated function user_roles():                              
         in drupal:10.2.0 and is removed from drupal:11.0.0. Use                
           \Drupal\user\Entity\Role::loadMultiple() and, if necessary, an       
         inline                                                                 
           implementation instead.                                              
  290    Method                                                                 
         Drupal\user_default_page\Form\UserDefaultPageConfigEntityForm::save()  
         should return int but return statement is missing.