Add JUnit 5 checkstyle rules

Add a rule to enforce JUnit 5 usage and conventions.

Closes gh-17093

spring-boot-project/spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/web/client/RestClientTestWithComponentIntegrationTests.java

@@ -37,6 +37,9 @@ import static org.springframework.test.web.client.response.MockRestResponseCreat
 @RestClientTest(ExampleRestClient.class)
 public class RestClientTestWithComponentIntegrationTests {
 
+	// JUnit 4 because RestClientTestWithoutJacksonIntegrationTests uses
+	// ModifiedClassPathRunner
+
 	@Autowired
 	private MockRestServiceServer server;