refactor: changed UserManager constructor to private

Server/src/main/java/uulm/teamname/marvelous/server/netconnector/UserManager.java

@@ -65,7 +65,7 @@ public class UserManager {
     private final JSON json;
 
     /** Constructs a new, empty UserManager */
-    UserManager() {
+    private UserManager() {
         this.newUsers = new HashSet<>();
         this.readyToConnect = new HashMap<>();
         this.readyToReconnect = new HashMap<>();