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/client4.go

@@ -2453,8 +2453,8 @@ func (c *Client4) ConvertChannelToPrivate(channelId string) (*Channel, *Response
 }
 
 // UpdateChannelPrivacy updates channel privacy
-func (c *Client4) UpdateChannelPrivacy(channelId string, privacy string) (*Channel, *Response) {
-	requestBody := map[string]string{"privacy": privacy}
+func (c *Client4) UpdateChannelPrivacy(channelId string, privacy ChannelType) (*Channel, *Response) {
+	requestBody := map[string]string{"privacy": string(privacy)}
 	r, err := c.DoApiPut(c.GetChannelRoute(channelId)+"/privacy", MapToJson(requestBody))
 	if err != nil {
 		return nil, BuildErrorResponse(r, err)