add DefaultBackground color

Author: kkuepper

Src/Notion.Client/Models/Blocks/Color.cs

@@ -36,6 +36,9 @@ public enum Color
         [EnumMember(Value = "red")]
         Red,
 
+        [EnumMember(Value = "default_background")]
+        DefaultBackground,
+        
         [EnumMember(Value = "gray_background")]
         GrayBackground,