Use score.User.OnlineID instead of score.UserID

You'd hope that they'd be the same thing, but
post-https://github.com/ppy/osu-server-spectator/pull/230 it turns out
that cannot be guaranteed, so just attempt to use `User` in the encoder
consistently everywhere...

osu.Game/Scoring/Legacy/LegacyReplaySoloScoreInfo.cs

@@ -54,7 +54,7 @@ namespace osu.Game.Scoring.Legacy
             MaximumStatistics = score.MaximumStatistics.Where(kvp => kvp.Value != 0).ToDictionary(),
             ClientVersion = score.ClientVersion,
             Rank = score.Rank,
-            UserID = score.UserID,
+            UserID = score.User.OnlineID,
         };
     }
 }