Fix code depreactions for phpstan 1.x (required for PHP 8.1)

[stephen-cox]

We need to upgrade phpstan to 1.x for PHP 8.1. This highlights the following issues:

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  Line   themes/contrib/localgov_theme/localgov_theme.theme                           
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  51     Call to deprecated function file_create_url():                               
         in drupal:9.3.0 and is removed from drupal:10.0.0.                           
           Use the appropriate method on \Drupal\Core\File\FileUrlGeneratorInterface  
           instead.                                                                   
  51     Call to deprecated function file_url_transform_relative():                   
         in drupal:9.3.0 and is removed from drupal:10.0.0.                           
           Use \Drupal\Core\File\FileUrlGenerator::transformRelative() instead.       
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