Add copyright for clouds

[RighthandSon]

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Content (Artwork / Missions / Jobs)

Summary

{{summarize your content! Include links to related issues, in-game screenshots, etc.}}
@Zitchas apparently forgot to give a copyright to the cloud images in #4871
Also couldn't find 2x for a couple so I redid them to match a new 2x

Save File

This save file can be used to play through the new mission content:
{{attach a save file that allows people to easily test your added mission content or see your new in-game art}}

PR Checklist

• I updated the copyright attributions, or decline to claim copyright of any assets produced or modified
• I uploaded the necessary image, blend, and texture assets here: {{insert link to assets}}
• I created a PR to the endless-sky-high-dpi repo with the @2x versions of these art assets: {{https://github.com/endless-sky/endless-sky-high-dpi/pull/153}}

[Zitchas]

Thanks for catching this, and sorry!