MM-21357: Use typed constant for channel types (#17928)

* MM-21357: Use typed constant for channel types

https://mattermost.atlassian.net/browse/MM-21357

```release-note
- Introduced a new type ChannelType for all channel types.
- Updated the Client4.UpdateChannelPrivacy method to ChannelType.
```

* Address review comments

```release-note
NONE
```

* telemetry fix

```release-note
NONE
```

model/message_export.go

@@ -11,7 +11,7 @@ type MessageExport struct {
 	ChannelId          *string
 	ChannelName        *string
 	ChannelDisplayName *string
-	ChannelType        *string
+	ChannelType        *ChannelType
 
 	UserId    *string
 	UserEmail *string