Fix boost warn: use boost::system instead boost::json error code

[ramon-bernardo]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

• Fix boost warn: warning C4996: 'boost::json::error_code': Use boost::system::error_code instead