I believe this is the media_directories/modules/media_directories_editor/images/folder.png
Status report shows an error: Embed button CKEditor5 compatiblity
Saying: The following embed button(s) are not using SVG icons and will not be compatible with CKEditor5 -- Media: /sites/default/files/embed_buttons/folder.png
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Converting to SVG
Active
2.2
Miscellaneous