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
```

app/import_validators_test.go

@@ -357,11 +357,12 @@ func TestImportValidateTeamImportData(t *testing.T) {
 func TestImportValidateChannelImportData(t *testing.T) {
 
 	// Test with minimum required valid properties.
+	chanTypeOpen := model.ChannelTypeOpen
 	data := ChannelImportData{
 		Team:        ptrStr("teamname"),
 		Name:        ptrStr("channelname"),
 		DisplayName: ptrStr("Display Name"),
-		Type:        ptrStr("O"),
+		Type:        &chanTypeOpen,
 	}
 	err := validateChannelImportData(&data)
 	require.Nil(t, err, "Validation failed but should have been valid.")
@@ -370,7 +371,7 @@ func TestImportValidateChannelImportData(t *testing.T) {
 	data = ChannelImportData{
 		Name:        ptrStr("channelname"),
 		DisplayName: ptrStr("Display Name"),
-		Type:        ptrStr("O"),
+		Type:        &chanTypeOpen,
 	}
 	err = validateChannelImportData(&data)
 	require.NotNil(t, err, "Should have failed due to missing team.")
@@ -379,7 +380,7 @@ func TestImportValidateChannelImportData(t *testing.T) {
 	data = ChannelImportData{
 		Team:        ptrStr("teamname"),
 		DisplayName: ptrStr("Display Name"),
-		Type:        ptrStr("O"),
+		Type:        &chanTypeOpen,
 	}
 	err = validateChannelImportData(&data)
 	require.NotNil(t, err, "Should have failed due to missing name.")
@@ -400,7 +401,7 @@ func TestImportValidateChannelImportData(t *testing.T) {
 	data = ChannelImportData{
 		Team: ptrStr("teamname"),
 		Name: ptrStr("channelname"),
-		Type: ptrStr("O"),
+		Type: &chanTypeOpen,
 	}
 	err = validateChannelImportData(&data)
 	require.NotNil(t, err, "Should have failed due to missing display_name.")
@@ -422,11 +423,13 @@ func TestImportValidateChannelImportData(t *testing.T) {
 	err = validateChannelImportData(&data)
 	require.NotNil(t, err, "Should have failed due to missing type.")
 
-	data.Type = ptrStr("A")
+	invalidType := model.ChannelType("A")
+	data.Type = &invalidType
 	err = validateChannelImportData(&data)
 	require.NotNil(t, err, "Should have failed due to invalid type.")
 
-	data.Type = ptrStr("P")
+	chanTypePr := model.ChannelTypePrivate
+	data.Type = &chanTypePr
 	err = validateChannelImportData(&data)
 	require.Nil(t, err, "Should have succeeded with valid type.")
 
@@ -435,7 +438,7 @@ func TestImportValidateChannelImportData(t *testing.T) {
 		Team:        ptrStr("teamname"),
 		Name:        ptrStr("channelname"),
 		DisplayName: ptrStr("Display Name"),
-		Type:        ptrStr("O"),
+		Type:        &chanTypeOpen,
 		Header:      ptrStr("Channel Header Here"),
 		Purpose:     ptrStr("Channel Purpose Here"),
 	}