PHPStan reports 2 static code issues

Problem/Motivation

Code should be clean and follow coding standards.

Steps to reproduce

When testing code with PHPStan following issues has been reported:

------ ----------------------------------------------------------------------- 
  Line   .../simple_sitemap_extensions/src/Plugin/simple_sitemap/UrlGene  
         rator/ExtendedEntityUrlGenerator.php                                   
 ------ ----------------------------------------------------------------------- 
  168    \Drupal calls should be avoided in classes, use dependency injection   
         instead                                                                
  245    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.                                                             
 ------ ----------------------------------------------------------------------- 

Proposed resolution

Code should be fixed.
Work has been started already at:
https://github.com/drunomics/simple-sitemap-extensions/pull/11/files

Remaining tasks

User interface changes

API changes

Data model changes