Add "F" ScoreRank to handle old scores which have this specified

Not sure on the future of this, but given it is used in the
save-failed-reply pull request (#18785) I think it's fine to add back
for now.

Without this, JSON parsing of older scores in server-side storage will
fail on missing enum type.

osu.Game/Scoring/ScoreRank.cs

@@ -11,6 +11,10 @@ namespace osu.Game.Scoring
 {
     public enum ScoreRank
     {
+        // TODO: Localisable?
+        [Description(@"F")]
+        F = -1,
+
         [LocalisableDescription(typeof(BeatmapsStrings), nameof(BeatmapsStrings.RankD))]
         [Description(@"D")]
         D,