If there's no authentication manager bean or no bean from which
one can be created, Spring Security's reactive support may fail to
bootstrap due to a null authentication manager.
This commit causes the auto-configuration that enables WebFlux
security to back off in the absence of an AuthenticationManager bean
and a ReactiveUserDetailsService (from which Spring Security can
create an AuthenticationManager) bean. Other reactive security
auto-configuration that can configure things such that WebFlux security
can be bootstrapped without an AuthenticationManager has been updated
to enable WebFlux security rather than relying on another
auto-configuration class to do so.
Fixes gh-37504
ee9c74556d