Update JUnit 5 annotations in documentation

- replace Before with BeforeEach
- replace RunWith with ExtendWith

Closes gh-10934

docs/modules/ROOT/pages/reactive/test/web/setup.adoc

@@ -4,7 +4,7 @@ The basic setup looks like this:
 
 [source,java]
 ----
-@RunWith(SpringRunner.class)
+@ExtendWith(SpringExtension.class)
 @ContextConfiguration(classes = HelloWebfluxMethodApplication.class)
 public class HelloWebfluxMethodApplicationTests {
 	@Autowired
@@ -12,7 +12,7 @@ public class HelloWebfluxMethodApplicationTests {
 
 	WebTestClient rest;
 
-	@Before
+	@BeforeEach
 	public void setup() {
 		this.rest = WebTestClient
 			.bindToApplicationContext(this.context)