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-redis/src/main/java/org/springframework/integration/redis/inbound/RedisQueueInboundGateway.java

@@ -19,8 +19,6 @@ package org.springframework.integration.redis.inbound;
 import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 
-import org.jetbrains.annotations.Nullable;
-
 import org.springframework.beans.factory.BeanClassLoaderAware;
 import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.ApplicationEventPublisherAware;
@@ -39,6 +37,7 @@ import org.springframework.integration.util.ErrorHandlingTaskExecutor;
 import org.springframework.jmx.export.annotation.ManagedMetric;
 import org.springframework.jmx.export.annotation.ManagedOperation;
 import org.springframework.jmx.export.annotation.ManagedResource;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessagingException;
 import org.springframework.scheduling.SchedulingAwareRunnable;