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-jdbc/src/test/kotlin/org/springframework/jdbc/core/JdbcOperationsExtensionsTests.kt

@@ -21,8 +21,8 @@ import java.sql.*
 import io.mockk.every
 import io.mockk.mockk
 import io.mockk.verify
-import org.junit.Assert.assertEquals
-import org.junit.Assert.assertNull
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Test
 
 /**

spring-jdbc/src/test/kotlin/org/springframework/jdbc/core/namedparam/MapSqlParameterSourceExtensionsTests.kt

@@ -18,7 +18,7 @@ package org.springframework.jdbc.core.namedparam
 
 import java.sql.JDBCType
 
-import org.junit.Assert.assertEquals
+import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 
 /**