Adopt GitLab CI: PHPStan compliance + test against 9.5/10.0/10.1/10.2/11.x + max PHP version

This blocks 🐛 Call to Deprecated Function 'file_url_transform_relative()' RTBC and 📌 Adopt Drupal core 10.3 config validation infrastructure Active .

The 2 remaining PHPStan errors:

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  Line   cdn.module                                                           
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  53     Function file_create_url not found.                                  
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols  
  53     Function file_url_transform_relative not found.                      
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols  
 ------ --------------------------------------------------------------------- 

will be fixed by 🐛 Call to Deprecated Function 'file_url_transform_relative()' RTBC .

Automatically closed - issue fixed for 2 weeks with no activity.