Improve POTD size in explore feed

[tonisevener]

Fixes Phabricator ticket: https://phabricator.wikimedia.org/T164642

Notes

• We added extra logic in https://github.com/wikimedia/wikipedia-ios/pull/3716/files to choose a clearer POTD url for displaying in the iOS14 widget. This ticket uses that same helper method to choose a better image URL for the explore feed card.

Test Steps

1. Map locally the https://en.wikipedia.org/api/rest_v1/feed/featured/2021/xx/xx endpoint (depending on which day you're testing) with the response at https://en.wikipedia.org/api/rest_v1/feed/featured/2017/05/04. That will allow you to directly test the image that the original bug was filed against.
2. General POTD explore feed regression testing without mapping locally.

Screenshots/Videos (if applicable)

Before:

After:

[staykids]

Nice! Offers a pinch higher quality. The only thing I wonder is if this increase in image bucket size will make any kind of noticeable difference for folks on slower connections. But in my basic iPhone/iPad testing so far works fine.