fix: send callback command to >= 1.21.6 clients (#1627)

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -359,7 +359,12 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
       // Inject commands from the proxy.
       final CommandGraphInjector<CommandSource> injector = server.getCommandManager().getInjector();
       injector.inject(rootNode, serverConn.getPlayer());
-      rootNode.removeChildByName("velocity:callback");
+
+      // In 1.21.6 a confirmation prompt was added when executing a command via `run_command` click
+      // action if the command is unknown. To prevent this prompt we have to send the command.
+      if (this.playerConnection.getProtocolVersion().lessThan(ProtocolVersion.MINECRAFT_1_21_6)) {
+        rootNode.removeChildByName("velocity:callback");
+      }
     }
 
     server.getEventManager().fire(