GH-3428: Make Kotlin dependency optional again

Fixes https://github.com/spring-projects/spring-integration/issues/3428

The Kotlin Gradle plugin starting with Kotlin `1.4` adds a standard
Kotlin lib into `compile` scope fully ignoring an `optional` variant.
See more info here: https://kotlinlang.org/docs/reference/using-gradle.html#dependency-on-the-standard-library

* Add `kotlin.stdlib.default.dependency=false` into `gradle.properties`
to rely only on the explicit dependency definitions in the project config
* Remove explicit Kotlin deps from test scope in the target modules since
it is declared globally for all the modules
* Fix author emails in the `publish-maven.gradle` to their VMware variants
* Remove usage of `org.jetbrains.annotations` in favor of `@Nullable`
from Spring Framework

spring-integration-stomp/src/main/java/org/springframework/integration/stomp/AbstractStompSessionManager.java

@@ -27,7 +27,6 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
-import org.jetbrains.annotations.NotNull;
 
 import org.springframework.beans.factory.BeanNameAware;
 import org.springframework.beans.factory.DisposableBean;
@@ -215,7 +214,6 @@ public abstract class AbstractStompSessionManager implements StompSessionManager
 		}
 	}
 
-	@NotNull
 	private CountDownLatch addStompSessionCallback(int currentEpoch) {
 		CountDownLatch connectLatch = new CountDownLatch(1);
 		this.stompSessionListenableFuture.addCallback(