Bug: updated low activity image to gray version for workspace OG image

[nickytonline]

          We'll also need to update this image that is used for workspace OG images. Can you provide a new one @isabensusan when you have a chance?

https://github.com/open-sauced/app/blob/c6e0514bc2c2dfa9d20be9f18d50662e1593c351/public/assets/og-images/workspaces/low-activity.png

Originally posted by @nickytonline in #3345 (comment)

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