Don't reattempt RichMediaBackfill by default

Retrying seems unlikely to be helpful:
 - if it timed out, chances are the short delay before reattempting
   won't give the remote enough time to recover from its outage and
   a longer delay makes the job pointless as users likely scrolled
   further already. (Likely this is already be the case after the
   first 20s timeout)
 - if the remote data is so borked we couldn't even parse it far enough
   for an "invalid metadata" error, chances are it will remain borked
   upon reattempt

config/config.exs

@@ -602,7 +602,7 @@
     federator_incoming: 5,
     federator_outgoing: 5,
     search_indexing: 2,
-    rich_media_backfill: 3
+    rich_media_backfill: 1
   ],
   timeout: [
     activity_expiration: :timer.seconds(5),