Update to Drupal 9
Root .theme file
Call to deprecated function
file_unmanaged_save_data(). Deprecated in drupal:8.7.0 and is removed from drupal:9.0.0. UseDrupal\Core\File\FileSystemInterface::saveData().
Info file:
Add
core_version_requirement: ^8 || ^9
(Checked the Umamy theme.)
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