EXISTS_REPLACE is deprecated [#3590912]

Problem/Motivation

Fetching deprecated class constant EXISTS_REPLACE of interface Drupal\Core\File\FileSystemInterface. Deprecated in drupal:10.3.0 and is removed from drupal:12.0.0. Use Drupal\Core\File\FileExists::Replace instead.

Proposed resolution

See https://www.drupal.org/node/3426517.

Issue fork css_editor-3590912

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2.0.x changes, plain diff MR !9
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