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
```

app/notification_push.go

@@ -161,7 +161,7 @@ func (a *App) sendPushNotification(notification *PostNotification, user *model.U
 }
 
 func (a *App) getPushNotificationMessage(contentsConfig, postMessage string, explicitMention, channelWideMention,
-	hasFiles bool, senderName, channelType, replyToThreadType string, userLocale i18n.TranslateFunc) string {
+	hasFiles bool, senderName string, channelType model.ChannelType, replyToThreadType string, userLocale i18n.TranslateFunc) string {
 
 	// If the post only has images then push an appropriate message
 	if postMessage == "" && hasFiles {