Update file_entity in composer.json

Problem/Motivation

I created the related issue for file_entity, to prevent errors when normalizing a file entity that has it's image/file deleted. A new release of file_entity was released today, 2.0-rc6. Right now WxT has file_entity pinned at 2.0-rc5. We should use a caret version range for this module so it can be updated without a change to WxT.