Seal most event hierarchies (#10515)

Most events in Forge are not designed to be extended by mod devs but technically allowed it before this PR, leading to a risk of unintentional breaking changes for those that do.

If you need to extend a Forge event, please let us know so that we can explicitly support that and design it with extending in mind. Note that this change does not affect making your own events that directly extend from EventBus' `Event` class.

src/main/java/net/minecraftforge/event/ServerChatEvent.java

@@ -26,7 +26,7 @@ import org.jetbrains.annotations.ApiStatus;
  * only on the {@linkplain LogicalSide#SERVER logical server}.
  **/
 @Cancelable
-public class ServerChatEvent extends Event {
+public final class ServerChatEvent extends Event {
     private final ServerPlayer player;
     private final String username;
     private final String rawText;