fix: allow demo for older versions

We were not propagating the '--demo' flag in the legacy launcher,
unconditionally setting the 'demo' parameter to false.

Signed-off-by: flow <flowlnlnln@gmail.com>

libraries/launcher/org/polymc/impl/OneSixLauncher.java

@@ -137,7 +137,8 @@ public final class OneSixLauncher implements Launcher {
                         winSizeH,
                         maximize,
                         serverAddress,
-                        serverPort
+                        serverPort,
+                        mcParams.contains("--demo")
                 );
 
                 return;