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
```

api4/channel_test.go

@@ -1850,7 +1850,7 @@ func TestUpdateChannelPrivacy(t *testing.T) {
 	type testTable []struct {
 		name            string
 		channel         *model.Channel
-		expectedPrivacy string
+		expectedPrivacy model.ChannelType
 	}
 
 	t.Run("Should get a forbidden response if not logged in", func(t *testing.T) {