Correct Json marshaling for labels of issues

Labels for issues should be marshalled to a list of strings instead of single (comma-seperated) string. This is according to https://docs.gitlab.com/ee/api/issues.html (22-11-2023)

issues.go

@@ -175,7 +175,7 @@ func (l *Labels) MarshalJSON() ([]byte, error) {
 	if *l == nil {
 		return []byte(`null`), nil
 	}
-	return json.Marshal(strings.Join(*l, ","))
+	return json.Marshal(*l)
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.