third-party: update oatpp to 1.3.1

[VDanielEdwards]

Subject

Update oatpp to version 1.3.1 which fixes a warning emitted on new-ish GCC versions.

[...]/oatpp/src/oatpp/network/monitor/ConnectionMonitor.cpp: In static member function ‘static void oatpp::network::monitor::ConnectionMonitor::Monitor::monitorTask(std::shared_ptr<oatpp::network::monitor::ConnectionMonitor::Monitor>)’:
[...]/oatpp/src/oatpp/network/monitor/ConnectionMonitor.cpp:146:52: warning:
ignoring return value of ‘std::lock_guard<_Mutex>::lock_guard(mutex_type&) [with _Mutex = std::mutex; mutex_type = std::mutex]’, declared with attribute ‘nodiscard’ [-Wunused-result]

Developer checklist (address before review)

• Changelog.md updated
• Prepared update for depending repositories
• Documentation updated (public API changes only)
• API docstrings updated (public API changes only)
• Rebase → commit history clean
• Squash and merge → proper PR title

[KonradBreitsprecherBkd]

LGTM