Align Resource Server documentation with Boot's capabilities

[wilkinsona]

Since 2.7.0 (spring-projects/spring-boot#29084), Boot's had support for auto-configuring a validator for the aud claim. It would be good if Spring Security's documentation could be updated to align with this. Validating the aud claim is currently used as an example of custom validation but you can now just set spring.security.oauth2.resourceserver.jwt.audiences instead.

spring-projects/spring-boot#34848 is tracking some corresponding updates to Boot's reference docs.