Remove Photon srcset from Plugin Directory screenshots

[dan-zakirov]

This removes the Photon based srcset for Plugin Directory screenshot thumbnails and keeps the images on ps.w.org.

The current src already points to ps.w.org with the SVN revision cachebuster, but production srcset can still route thumbnail candidates through i0.wp.com. That adds a second CDN cache layer and can make replaced screenshots appear stale for some users.

This keeps the existing Gallery block output, lightbox behavior, captions, layout detection, dimensions, and reveal behavior unchanged.

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[medavidallsop]

I also noticed this on a few plugins. Added info to Slack:

On a plugin, I updated the screenshot assets, im seeing after 48 hours the thumbnails on the plugin page still haven't updated, but if you enlarge the screenshots they show the updated images - the enlarged screenshots come from ps.w.org and they are fine, but the thumbs come from i0.wp.com/ps.w.org and those remain the old ones

https://wordpress.slack.com/archives/C02QB8GMM/p1784192370772129