Add an option for disabling the legacy AEW&C aircraft.

Using the legacy AEW&C aircraft is still the default until
https://github.com/Khopa/dcs_liberation/issues/844 is fixed.

game/settings.py

@@ -30,6 +30,7 @@ class Settings:
     automate_front_line_reinforcements: bool = False
     automate_aircraft_reinforcements: bool = False
     restrict_weapons_by_date: bool = False
+    disable_legacy_aewc: bool = False
 
     # Performance oriented
     perf_red_alert_state: bool = True