reserve_loopback_port() in 2.4.0 still declares the removed boost::asio::io_service
type (dropped in Boost 1.87+, succeeded by io_context). Every other asio user in
the tree already uses io_context; this lone holdout breaks the build against modern
Boost. Switch the type to io_context (keeping the local variable name io_service so
the acceptor ctor on the next line is untouched). io_context has the same length, so
the column alignment is preserved.

--- a/src/slic3r/GUI/WebUserLoginDialog.cpp
+++ b/src/slic3r/GUI/WebUserLoginDialog.cpp
@@ -49,6 +49,6 @@
 {
     try {
-        boost::asio::io_service       io_service;
+        boost::asio::io_context       io_service;
         boost::asio::ip::tcp::acceptor acceptor(io_service, {boost::asio::ip::tcp::v4(), 0});
         return static_cast<int>(acceptor.local_endpoint().port());
     } catch (...) {