Document usage of VaadinSecurityConfigurer for Spring Security configuration

[mcollovati]

vaadin/flow#21373 introduced VaadinSecurityConfigurer for a better integration and modularization of Spring Security in Vaadin applications.

VaadinSecurityConfigurer will be the new recommended way to configure security, so its usage needs to be documented providing sensible example snippets.

The existing VaadinWebSecurity is supposed to be deprecated soon and then removed in the future versions, so its specific documentation should be moved to a separated paragraph highlighting that the feature is obsolete and will be removed. A migration example should be provided.
All other code snippets and references must be updated to use VaadinSecurityConfigurer.

Important: document the need to manually import VaadinAwareSecurityContextHolderStrategyConfiguration; see vaadin/flow#21356 (comment)