add the SystemChannelMessageDeny

unsure if there would be a better name for this enum, given it's inverted nature, open for suggestions

src/Discord.Net.Core/Entities/Guilds/GuildProperties.cs

@@ -70,5 +70,10 @@ namespace Discord
         ///     Gets or sets the explicit content filter level of this guild.
         /// </summary>
         public Optional<ExplicitContentFilterLevel> ExplicitContentFilter { get; set; }
+        /// <summary>
+        ///     Gets or sets the flags that DISABLE types of system channels.
+        ///     This logic is inverted!
+        /// </summary>
+        public Optional<SystemChannelMessageDeny> SystemChannelFlags { get; set; }
     }
 }

src/Discord.Net.Core/Entities/Guilds/SystemChannelMessageDeny.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace Discord
+{
+    [Flags]
+    public enum SystemChannelMessageDeny
+    {
+        /// <summary>
+        ///     The messages that are sent when a user joins the guild.
+        /// </summary>
+        WelcomeMessage = 0b1,
+        /// <summary>
+        ///     The messages that are sent when a user boosts the guild.
+        /// </summary>
+        GuildBoost = 0b10
+    }
+}