Fix WebSessionManager auto-configuration order

Auto-configure `SessionAutoConfiguration` before
`WebFluxAutoConfiguration` to ensure that the correct
`WebSessionManager` bean is created.

See gh-20970

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/SessionAutoConfiguration.java

@@ -42,6 +42,7 @@ import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
 import org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.ServerProperties;
 import org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration;
+import org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.boot.context.properties.PropertyMapper;
 import org.springframework.boot.web.servlet.server.Session.Cookie;
@@ -79,7 +80,7 @@ import org.springframework.session.web.http.HttpSessionIdResolver;
 @AutoConfigureAfter({ DataSourceAutoConfiguration.class, HazelcastAutoConfiguration.class,
 		JdbcTemplateAutoConfiguration.class, MongoDataAutoConfiguration.class, MongoReactiveDataAutoConfiguration.class,
 		RedisAutoConfiguration.class, RedisReactiveAutoConfiguration.class })
-@AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
+@AutoConfigureBefore({ HttpHandlerAutoConfiguration.class, WebFluxAutoConfiguration.class })
 public class SessionAutoConfiguration {
 
 	@Configuration(proxyBeanMethods = false)