Fix the PHPSTAN issue in the 3.x branch

Problem/Motivation

Now that we have a different branch (3.x) to support D11.2 and D12 and above, we can finally adress the PHPSTAN deprecation issue: https://git.drupalcode.org/project/media_thumbnails_svg/-/jobs/6684776

------ ---------------------------------------------------------------- 
  Line   src/Plugin/MediaThumbnail/MediaThumbnailSVG.php                 
 ------ ---------------------------------------------------------------- 
  82     Fetching deprecated class constant EXISTS_REPLACE of interface  
         Drupal\Core\File\FileSystemInterface:                           
         in drupal:10.3.0 and is removed from drupal:12.0.0. Use         
         \Drupal\Core\File\FileExists::Replace instead.                  
         🪪  classConstant.deprecated                                    
 ------ ---------------------------------------------------------------- 

Steps to reproduce

Look for the results of the pipeline execution for more details: https://git.drupalcode.org/project/media_thumbnails_svg/-/pipelines/610972

Proposed resolution

Fix the PHPSTAN issue.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A