Add JUnit 5 checkstyle rules

Add a rule to enforce JUnit 5 usage and conventions.

Closes gh-17093

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/SampleActuatorApplicationTests.java

@@ -166,7 +166,7 @@ class SampleActuatorApplicationTests {
 
 	@Test
 	@SuppressWarnings("unchecked")
-	public void testBeans() {
+	void testBeans() {
 		@SuppressWarnings("rawtypes")
 		ResponseEntity<Map> entity = this.restTemplate.withBasicAuth("user", getPassword())
 				.getForEntity("/actuator/beans", Map.class);

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/ShutdownSampleActuatorApplicationTests.java

@@ -58,7 +58,7 @@ class ShutdownSampleActuatorApplicationTests {
 
 	@Test
 	@DirtiesContext
-	public void testShutdown() {
+	void testShutdown() {
 		@SuppressWarnings("rawtypes")
 		ResponseEntity<Map> entity = this.restTemplate.withBasicAuth("user", getPassword())
 				.postForEntity("/actuator/shutdown", null, Map.class);