transport: call update_scheduling_group for non-auth connections

Before this change, unauthorized connections stayed in `main`
scheduling group. It is not ideal, in such case, rather `sl:default`
should be used, to have a consistent behavior with a scenario
where users is authenticated but there is no service level assigned
to the user.

This commit adds a call to `update_scheduling_group` at the end of
connection creation for an unauthenticated user, to make sure the
service level is switched to `sl:default`.

Fixes: scylladb/scylladb#26040

transport/server.cc

@@ -983,6 +983,7 @@ future<std::unique_ptr<cql_server::response>> cql_server::connection::process_st
             res = make_autheticate(stream, a.qualified_java_name(), trace_state);
         }
     } else {
+        update_scheduling_group();
         _ready = true;
         on_connection_ready();
         res = make_ready(stream, trace_state);