SPIGOT-4647: Properly cancel VehicleDamageEvent

By: md_5 <git@md-5.net>

paper-server/nms-patches/EntityMinecartAbstract.patch

@@ -46,7 +46,7 @@
 +                this.world.getServer().getPluginManager().callEvent(event);
 +
 +                if (event.isCancelled()) {
-+                    return true;
++                    return false;
 +                }
 +
 +                f = (float) event.getDamage();