message: Only allow a single "effects" node per `tech` node
  This is probably what was intended, even though the previous code would
  have allowed for multiple "effects" nodes, each with their own
  "modify" and "unlock" nodes.
diff: @@ -111,7 +111,7 @@ namespace
  tech.requiredTechnologies.push_back(std::stoi(id));
  }
  - for (auto techElement = technology.firstChildElement("effects"); techElement; techElement = techElement->nextSiblingElement("effects"))
  + if (const auto* techElement = technology.firstChildElement("effects"))
  {
  verifySubElementTypes(*techElement, {"modifier", "unlock"}, "TechnologyReader: ");
  