Add JUnit 5 checkstyle rules

Add a rule to enforce JUnit 5 usage and conventions.

Closes gh-17093

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/data/cassandra/CassandraDataAutoConfigurationIntegrationTests.java

@@ -71,7 +71,6 @@ class CassandraDataAutoConfigurationIntegrationTests {
 		AutoConfigurationPackages.register(this.context, cityPackage);
 		this.context.register(CassandraAutoConfiguration.class, CassandraDataAutoConfiguration.class);
 		this.context.refresh();
-
 		CassandraSessionFactoryBean bean = this.context.getBean(CassandraSessionFactoryBean.class);
 		assertThat(bean.getSchemaAction()).isEqualTo(SchemaAction.NONE);
 	}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/data/elasticsearch/ElasticsearchDataAutoConfigurationTests.java

@@ -103,7 +103,7 @@ class ElasticsearchDataAutoConfigurationTests {
 	}
 
 	@Test
-	public void customReactiveRestTemplateShouldBeUsed() {
+	void customReactiveRestTemplateShouldBeUsed() {
 		this.contextRunner.withUserConfiguration(CustomReactiveRestTemplate.class)
 				.run((context) -> assertThat(context).getBeanNames(ReactiveElasticsearchTemplate.class).hasSize(1)
 						.contains("reactiveElasticsearchTemplate"));

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/rsocket/RSocketMessagingAutoConfigurationTests.java

@@ -42,7 +42,7 @@ class RSocketMessagingAutoConfigurationTests {
 			.withUserConfiguration(BaseConfiguration.class);
 
 	@Test
-	public void shouldCreateDefaultBeans() {
+	void shouldCreateDefaultBeans() {
 		this.contextRunner.run((context) -> {
 			assertThat(context).getBeans(MessageHandlerAcceptor.class).hasSize(1);
 			assertThat(context.getBean(MessageHandlerAcceptor.class).getRouteMatcher())