Prepare the module for D9

The module has a few uses of deprecated APIs:

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  Line   contrib/views_show_more/views_show_more.module                                        
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  24     Call to deprecated function pager_query_add_page():                                   
         in drupal:8.8.0 and is removed from drupal:9.0.0. Use                                 
         \Drupal\Core\Pager\PagerManagerInterface::getUpdatedParameters() instead.             
  28     Call to deprecated method url() of class Drupal:                                      
         in drupal:8.0.0 and is removed from drupal:9.0.0.                                     
         Instead create a \Drupal\Core\Url object directly, for example using                  
         Url::fromRoute().                                                                     
  35     Call to deprecated method checkPlain() of class Drupal\Component\Utility\SafeMarkup:  
         Will be removed before Drupal 9.0.0. Rely on Twig's                                   
         auto-escaping feature, or use the @link theme_render #plain_text @endlink             
         key when constructing a render array that contains plain text in order to             
         use the renderer's auto-escaping feature. If neither of these are                     
         possible, \Drupal\Component\Utility\Html::escape() can be used in places              
         where explicit escaping is needed.                                                    
  35     Call to method checkPlain() of deprecated class Drupal\Component\Utility\SafeMarkup:  
         Will be removed before Drupal 9.0.0. Use the appropriate                              
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