feature: Disconnect guild members from voice channels (#1311)

* Feature: Disconnect users from voice channels

Updates GuildUserProperties to allow for setting either Channel or ChannelId to null, which will disconnect users from voice channels.

The type of ChannelId has been updated from a ulong to ulong?, which matches the latest api documentation.

* docs: update xmldoc wording

* breaking workaround, revert ChannelId to ulong

This is a workaround to prevent this PR from being a breaking
change. Reverts the type of GuildUserProperties#ChannelId to a
ulong from a ulong?. Guild Users may no longer be kicked by
setting this property to null, but setting
GuildUserProperties#Channel should still work.

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

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API.Rest
@@ -15,6 +15,6 @@ namespace Discord.API.Rest
         [JsonProperty("roles")]
         public Optional<ulong[]> RoleIds { get; set; }
         [JsonProperty("channel_id")]
-        public Optional<ulong> ChannelId { get; set; }
+        public Optional<ulong?> ChannelId { get; set; }
     }
 }