Add handling for legacy CatchTheBeat section in LegacyDecoder

osu.Game/Beatmaps/Formats/LegacyDecoder.cs

@@ -73,6 +73,9 @@ protected virtual void ParseLine(T output, Section section, string line)
             switch (section)
             {
                 case Section.Colours:
+                // osu!catch section only has colour settings
+                // so no harm in handling the entire section
+                case Section.CatchTheBeat:
                     HandleColours(output, line);
                     return;
             }
@@ -149,7 +152,8 @@ protected enum Section
             HitObjects,
             Variables,
             Fonts,
-            Mania
+            CatchTheBeat,
+            Mania,
         }
 
         internal class LegacyDifficultyControlPoint : DifficultyControlPoint