Document that JUnit 5 is the default

See gh-14736

spring-boot-project/spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -623,7 +623,6 @@ shown in the following example:
 
 [source,java,indent=0,subs="verbatim,quotes,attributes"]
 ----
-	@RunWith(SpringJUnit4ClassRunner.class)
 	@SpringBootTest(webEnvironment=WebEnvironment.RANDOM_PORT)
 	public class MyWebIntegrationTests {
 

spring-boot-samples/spring-boot-sample-junit-jupiter/pom.xml

@@ -25,21 +25,11 @@
 			<scope>test</scope>
 			<exclusions>
 				<exclusion>
-					<groupId>junit</groupId>
-					<artifactId>junit</artifactId>
+					<groupId>org.junit.vintage</groupId>
+					<artifactId>junit-vintage-engine</artifactId>
 				</exclusion>
 			</exclusions>
 		</dependency>
-		<dependency>
-			<groupId>org.junit.jupiter</groupId>
-			<artifactId>junit-jupiter-api</artifactId>
-			<scope>test</scope>
-		</dependency>
-		<dependency>
-			<groupId>org.junit.jupiter</groupId>
-			<artifactId>junit-jupiter-engine</artifactId>
-			<scope>test</scope>
-		</dependency>
 	</dependencies>
 	<build>
 		<plugins>