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-test/spring-test.gradle

@@ -2,6 +2,12 @@ description = "Spring TestContext Framework"
 
 apply plugin: "kotlin"
 
+// Workaround for https://youtrack.jetbrains.com/issue/KT-39610
+configurations["optional"].attributes.attribute(
+		org.gradle.api.attributes.Usage.USAGE_ATTRIBUTE,
+		objects.named(Usage.class, "java-runtime")
+)
+
 dependencies {
 	compile(project(":spring-core"))
 	optional(project(":spring-aop"))