Update webflux-form sample to use Built in CSRF Support

Remove the CsrfControllerAdvice class and update dependencies to add
org.thymeleaf.extras:thymeleaf-extras-springsecurity5

Issue: gh-6061

samples/boot/webflux-form/spring-security-samples-boot-webflux-form.gradle

@@ -22,6 +22,8 @@ dependencies {
 	compile project(':spring-security-web')
 	compile 'org.springframework.boot:spring-boot-starter-thymeleaf'
 	compile 'org.springframework.boot:spring-boot-starter-webflux'
+	compile 'org.thymeleaf.extras:thymeleaf-extras-springsecurity5'
+
 
 	testCompile project(':spring-security-test')
 	testCompile 'org.springframework.boot:spring-boot-starter-test'

samples/boot/webflux-form/src/main/java/sample/CsrfControllerAdvice.java

@@ -1,38 +0,0 @@
-/*
- * Copyright 2002-2017 the original author or authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package sample;
-
-import org.springframework.security.web.server.csrf.CsrfToken;
-import org.springframework.web.bind.annotation.ControllerAdvice;
-import org.springframework.web.bind.annotation.ModelAttribute;
-import org.springframework.web.server.ServerWebExchange;
-import reactor.core.publisher.Mono;
-
-import static org.springframework.security.web.reactive.result.view.CsrfRequestDataValueProcessor.DEFAULT_CSRF_ATTR_NAME;
-
-/**
- * @author Rob Winch
- * @since 5.0
- */
-@ControllerAdvice
-public class CsrfControllerAdvice {
-	@ModelAttribute
-	public Mono<CsrfToken> csrfToken(ServerWebExchange exchange) {
-		Mono<CsrfToken> csrfToken = exchange.getAttribute(CsrfToken.class.getName());
-		return csrfToken.doOnSuccess(token -> exchange.getAttributes().put(DEFAULT_CSRF_ATTR_NAME, token));
-	}
-}