Explicitly configure SecurityWebFilterChain bean for reactive oauth2 client

This will ensure that ReactiveManagementWebSecurityAutoConfiguration backs
off and that the actuator endpoints are also secured via OAuth2.

Fixes gh-17949

spring-boot-samples/spring-boot-sample-reactive-oauth2-client/src/test/java/sample/oauth2/client/SampleReactiveOAuth2ClientApplicationTests.java

@@ -51,4 +51,10 @@ public class SampleReactiveOAuth2ClientApplicationTests {
 		assertThat(bodyString).contains("/oauth2/authorization/github-client-2");
 	}
 
+	@Test
+	public void actuatorShouldBeSecuredByOAuth() {
+		this.webTestClient.get().uri("/actuator/health").exchange().expectStatus().isFound().expectHeader()
+				.valueEquals("Location", "/login");
+	}
+
 }