FIX: Add thumbnails for Vimeo unlisted lazy videos (#21343)

Previously, the image for the thumbnail was taken from the oembed endpoint.
Since these images are not available for some unlisted videos,
the thumbnails are now sourced from Open Graph instead.

plugins/discourse-lazy-videos/lib/lazy-videos/lazy_vimeo.rb

@@ -19,7 +19,7 @@ class Onebox::Engine::VimeoOnebox
         iframe_id = iframe_src.sub("https://player.vimeo.com/video/", "")
       end
 
-      thumbnail_url = "https://vumbnail.com/#{oembed_data[:video_id]}.jpg"
+      thumbnail_url = get_opengraph.image
       escaped_title = ERB::Util.html_escape(og_data.title)
 
       <<~HTML