Upgrade to Kotlin 1.4 M2

- The compiler is configured to retain compatibility with Kotlin 1.3.
- Explicit API mode is not yet enabled but could be in the future.
- A workaround for Gradle build is required for now, see
  https://youtrack.jetbrains.com/issue/KT-39610 for more details.
- Some exceptions thrown by Kotlin have changed to NullPointerException,
  see https://youtrack.jetbrains.com/issue/KT-22275 for more details.

Closes gh-24171

spring-messaging/src/test/kotlin/org/springframework/messaging/simp/annotation/support/SimpAnnotationMethodMessageHandlerKotlinTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020  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.
@@ -132,7 +132,7 @@ class SimpAnnotationMethodMessageHandlerKotlinTests {
         messageHandler.registerHandler(testController)
         messageHandler.handleMessage(message)
         assertThat(testController.exception).isNotNull()
-		assertThat(testController.exception).isInstanceOf(IllegalArgumentException::class.java)
+		assertThat(testController.exception).isInstanceOf(NullPointerException::class.java)
     }
 
     private fun createMessage(destination: String, headers: Map<String, String?>): Message<ByteArray> {