Add E-2D variant of the E-2C.

In game this is an E-2D, but the ID of the aircraft in the game data is
E-2C. Presumably it was repainted at some point in a DCS update.

This adds a variant but doesn't delete the old one to avoid breaking
campaigns and factions. I moved blufor modern to the E-2D but the rest
of the factions are too old so we'll let them pretend.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1696

resources/factions/bluefor_modern.json

@@ -33,7 +33,7 @@
     "UH-1H Iroquois"
   ],
   "awacs": [
-    "E-2C Hawkeye",
+    "E-2D Advanced Hawkeye",
     "E-3A"
   ],
   "tankers": [

resources/units/aircraft/E-2C.yaml

@@ -13,3 +13,4 @@ patrol:
   altitude: 30000
 variants:
   E-2C Hawkeye: {}
+  E-2D Advanced Hawkeye: {}