Remove JUnit 4 dependency from all modules except spring-test

This commit removes the JUnit 4 dependency from all modules except
spring-test which provides explicit JUnit 4 support.

This commit also includes the following.

- migration from JUnit 4 assertions to JUnit Jupiter assertions in all
  Kotlin tests
- migration from JUnit 4 assumptions in Spring's TestGroup support to
  JUnit Jupiter assumptions, based on org.opentest4j.TestAbortedException
- introduction of a new TestGroups utility class than can be used from
  existing JUnit 4 tests in the spring-test module in order to perform
  assumptions using JUnit 4's Assume class

See gh-23451

spring-messaging/src/test/kotlin/org/springframework/messaging/rsocket/RSocketRequesterExtensionsTests.kt

@@ -6,7 +6,7 @@ import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.runBlocking
-import org.junit.Assert.assertEquals
+import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.mockito.ArgumentMatchers.anyInt
 import org.reactivestreams.Publisher

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

@@ -36,7 +36,7 @@ import org.springframework.messaging.simp.SimpMessagingTemplate
 import org.springframework.messaging.support.MessageBuilder
 import org.springframework.stereotype.Controller
 
-import org.junit.Assert.*
+import org.junit.jupiter.api.Assertions.*
 import org.springframework.messaging.MessageHandlingException
 import org.springframework.messaging.handler.annotation.MessageExceptionHandler