Spring Security ReactiveOAuth2AuthorizedClientManagerConfiguration has been created too early

The Boot team is seeing the following warnings when running Spring Security related tests:

2024-04-12T12:07:39.300+01:00  WARN   --- [           main] o.s.c.a.ConfigurationClassPostProcessor  : Cannot enhance @Configuration bean definition 'org.springframework.security.config.annotation.web.reactive.ReactiveOAuth2ClientConfiguration$ReactiveOAuth2AuthorizedClientManagerConfiguration' since its singleton instance has been created too early. The typical cause is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor return type: Consider declaring such methods as 'static' and/or marking the containing configuration class as 'proxyBeanMethods=false'.

We should ensure the @Configuration class is marked proxyBeanMethods=false.