Raise log level to warn when configuration class enhancement cannot apply due to circular reference

[jhoeller]

Our existing log message for a circular reference to the configuration class is not ideally worded since it suggests that the configuration class cannot be enhanced whereas it actually can be enhanced but effectively won't use the enhanced class. Also, it suggests declaring the affected @Bean method as static but could also mention that the configuration class should be declared as proxyBeanMethods=false if enhancement is not needed anyway.

Most importantly, that log message can easily be missed at info level, so we should raise it to warn level.

Related PRs: #22992, #24356.