Enhancement: automatically clear the thumbnail cache for the post whenever its featured image is deleted or changed

Currently, when a post's post thumbnail (featured image) is deleted or changed, the cached Mini Loops thumbnail image data isn't automatically refreshed.

Adding `cache='clear' `to the mini loops shortcode/function can force the image thumbnail(s) to be rebuilt, but this is confusing to new users.

This enhancement automatically detects changes to a post's featured image, and regenerates the mini loops image(s) without the user needing to use cache=clear.