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
```

migrations/helper_test.go

@@ -159,7 +159,7 @@ func (th *TestHelper) CreateChannel(team *model.Team) *model.Channel {
 	return th.createChannel(team, model.ChannelTypeOpen)
 }
 
-func (th *TestHelper) createChannel(team *model.Team, channelType string) *model.Channel {
+func (th *TestHelper) createChannel(team *model.Team, channelType model.ChannelType) *model.Channel {
 	id := model.NewId()
 
 	channel := &model.Channel{