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
```

cmd/mattermost/commands/sampledata.go

@@ -617,9 +617,9 @@ func createChannel(idx int, teamName string) app.LineImportData {
 		purpose = purpose[0:250]
 	}
 
-	channelType := "P"
+	channelType := model.ChannelTypePrivate
 	if rand.Intn(2) == 0 {
-		channelType = "O"
+		channelType = model.ChannelTypeOpen
 	}
 
 	channel := app.ChannelImportData{