Add warnings to ancient deprecated Banners.json fields

Will probably turn these into runtime errors in 1.5.0, people need to stop sharing configs with dead fields and especially stop making PRs with them.

src/main/java/emu/grasscutter/game/gacha/GachaSystem.java

@@ -73,6 +73,7 @@ public class GachaSystem extends BaseGameSystem {
             List<GachaBanner> banners = DataLoader.loadList("Banners.json", GachaBanner.class);
             if (banners.size() > 0) {
                 for (GachaBanner banner : banners) {
+                    banner.onLoad();
                     getGachaBanners().put(banner.getScheduleId(), banner);
                 }
                 Grasscutter.getLogger().debug("Banners successfully loaded.");