fix: Update maximum value for SlowModeInterval (#1320)

Discord allows the slow mode to be up to 6 hours.

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -376,7 +376,7 @@ namespace Discord.API
             Preconditions.AtLeast(args.Position, 0, nameof(args.Position));
             Preconditions.NotNullOrEmpty(args.Name, nameof(args.Name));
             Preconditions.AtLeast(args.SlowModeInterval, 0, nameof(args.SlowModeInterval));
-            Preconditions.AtMost(args.SlowModeInterval, 120, nameof(args.SlowModeInterval));
+            Preconditions.AtMost(args.SlowModeInterval, 21600, nameof(args.SlowModeInterval));
             options = RequestOptions.CreateOrClone(options);
 
             var ids = new BucketIds(channelId: channelId);