Bump to Forge 1.12.2

Nobody is still using 1.12, so we might as well update to the latest -
it makes it easier to test mod compatibility at least.

src/main/java/dan200/computercraft/shared/turtle/recipes/TurtleUpgradeRecipe.java

@@ -37,7 +37,7 @@ public class TurtleUpgradeRecipe extends IForgeRegistryEntry.Impl<IRecipe> imple
     }
 
     @Override
-    public boolean isHidden()
+    public boolean isDynamic()
     {
         return true;
     }