implement the new modify guild params

src/Discord.Net.Rest/API/Rest/ModifyGuildParams.cs

@@ -30,5 +30,7 @@ namespace Discord.API.Rest
         public Optional<ulong> OwnerId { get; set; }
         [JsonProperty("explicit_content_filter")]
         public Optional<ExplicitContentFilterLevel> ExplicitContentFilter { get; set; }
+        [JsonProperty("system_channel_flags")]
+        public Optional<SystemChannelMessageDeny> SystemChannelFlags { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -33,7 +33,8 @@ namespace Discord.Rest
                 Name = args.Name,
                 Splash = args.Splash.IsSpecified ? args.Splash.Value?.ToModel() : Optional.Create<ImageModel?>(),
                 VerificationLevel = args.VerificationLevel,
-                ExplicitContentFilter = args.ExplicitContentFilter
+                ExplicitContentFilter = args.ExplicitContentFilter,
+                SystemChannelFlags = args.SystemChannelFlags
             };
 
             if (args.AfkChannel.IsSpecified)
@@ -64,6 +65,9 @@ namespace Discord.Rest
             if (args.ExplicitContentFilter.IsSpecified)
                 apiArgs.ExplicitContentFilter = args.ExplicitContentFilter.Value;
 
+            if (args.SystemChannelFlags.IsSpecified)
+                apiArgs.SystemChannelFlags = args.SystemChannelFlags.Value;
+
             return await client.ApiClient.ModifyGuildAsync(guild.Id, apiArgs, options).ConfigureAwait(false);
         }
         /// <exception cref="ArgumentNullException"><paramref name="func"/> is <c>null</c>.</exception>