AbstractUserDetailsReactiveAuthenticationManager uses boundidElastic()

Some JVMs have blocking operations when accessing SecureRandom and thus
this needs to be performed in a pool that is larger than the number of
CPUs

Closes gh-7522

Author: rwinch

core/src/main/java/org/springframework/security/authentication/AbstractUserDetailsReactiveAuthenticationManager.java

@@ -55,7 +55,7 @@ public abstract class AbstractUserDetailsReactiveAuthenticationManager implement
 
 	private ReactiveUserDetailsPasswordService userDetailsPasswordService;
 
-	private Scheduler scheduler = Schedulers.newParallel("password-encoder", Schedulers.DEFAULT_POOL_SIZE, true);
+	private Scheduler scheduler = Schedulers.boundedElastic();
 
 	private UserDetailsChecker preAuthenticationChecks = user -> {
 		if (!user.isAccountNonLocked()) {