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
```

services/slackimport/parsers.go

@@ -7,10 +7,11 @@ import (
 	"encoding/json"
 	"io"
 
+	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
 )
 
-func slackParseChannels(data io.Reader, channelType string) ([]slackChannel, error) {
+func slackParseChannels(data io.Reader, channelType model.ChannelType) ([]slackChannel, error) {
 	decoder := json.NewDecoder(data)
 
 	var channels []slackChannel

services/slackimport/slackimport.go

@@ -31,7 +31,7 @@ type slackChannel struct {
 	Members []string        `json:"members"`
 	Purpose slackChannelSub `json:"purpose"`
 	Topic   slackChannelSub `json:"topic"`
-	Type    string
+	Type    model.ChannelType
 }
 
 type slackChannelSub struct {