Blog thumbnails on use case pages

[zlonko]

Arrays for BlogListItems were updated in #5271, but not on the Incident Response and Code Health Use Case pages. This caused the thumbnails for blog items not to render. This updates the array from {...image} to the new structure, {...img:{src,alt}}.

[bretthayes]

Thanks for catching this @zlonko!! I saw this in one of my merge fixes in sourcegraph/about-replatform#144 so it's fixed in the replatform repo.