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
```

store/searchtest/helper.go

@@ -239,7 +239,7 @@ func (th *SearchTestHelper) deleteBot(botID string) error {
 	return nil
 }
 
-func (th *SearchTestHelper) createChannel(teamID, name, displayName, purpose, channelType string, deleted bool) (*model.Channel, error) {
+func (th *SearchTestHelper) createChannel(teamID, name, displayName, purpose string, channelType model.ChannelType, deleted bool) (*model.Channel, error) {
 	channel, err := th.Store.Channel().Save(&model.Channel{
 		TeamId:      teamID,
 		DisplayName: displayName,