Consider erroring when client authentication method is basic

[jzheaux]

Questions like https://stackoverflow.com/questions/75224023/spring-oauth2-login-not-working-after-migrating-to-spring-boot-3 indicate that it's not clear that clientAuthenticationMethod: basic is no longer a supported value for ClientAuthenticationMethod.

We should consider erroring and telling applications to change basic to client_secret_basic and post to client_secret_post.

(Note: this could also be done in Spring Boot possibly, though I think we should first consider doing it in Spring Security since we can catch more misconfigurations that way)

Related to https://docs.spring.io/spring-security/reference/5.8/migration/servlet/oauth2.html#_clientauthenticationmethod