Call to deprecated function file_unmanaged_save_data()

Problem/Motivation

Running drupal-check on the module getting the list of errors including next:

 ------ --------------------------------------------------------------------------------------- 
  Line   high_contrast.module                                                                   
 ------ --------------------------------------------------------------------------------------- 
  38     Call to deprecated function file_unmanaged_save_data():                                
         in drupal:8.7.0 and is removed from drupal:9.0.0.                                      
         Use \Drupal\Core\File\FileSystemInterface::saveData().                                 

Proposed resolution

See https://www.drupal.org/node/3006851

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet