Switch to non-blocking call in action

- UpgradeDeployTargetAppsSucceedAction can't use
  old non-reactive api's as we can't block within
  sm execution.

spring-cloud-skipper/spring-cloud-skipper-server-core/src/main/java/org/springframework/cloud/skipper/server/statemachine/UpgradeDeployTargetAppsSucceedAction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,8 +15,12 @@
  */
 package org.springframework.cloud.skipper.server.statemachine;
 
+import reactor.core.publisher.Mono;
+
 import org.springframework.cloud.skipper.server.statemachine.SkipperStateMachineService.SkipperEvents;
 import org.springframework.cloud.skipper.server.statemachine.SkipperStateMachineService.SkipperStates;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.statemachine.StateContext;
 import org.springframework.statemachine.action.Action;
 
@@ -32,6 +36,10 @@ public class UpgradeDeployTargetAppsSucceedAction extends AbstractAction {
 	protected void executeInternal(StateContext<SkipperStates, SkipperEvents> context) {
 		// TODO: when we support other type of strategies, we would not need to just
 		//       blindly send accept as we can also cancel upgrade in this stage.
-		context.getStateMachine().sendEvent(SkipperEvents.UPGRADE_ACCEPT);
+		Message<SkipperEvents> event = MessageBuilder.withPayload(SkipperEvents.UPGRADE_ACCEPT).build();
+		// TODO: blindly sending event and subscribing(we can't block here) into it is not
+		//       super nice, should look better reactive chain possibly better
+		//       reactive support in a statemachine
+		context.getStateMachine().sendEvent(Mono.just(event)).subscribe();
 	}
 }