Update What's New in 6.0 for PasswordEncoders

Issue gh-11985

Author: jgrandja

docs/modules/ROOT/pages/whats-new.adoc

@@ -28,3 +28,4 @@ If application relies on the implicit `<intercept-url pattern="/**" access="perm
 Or use `use-authorization-manager="false"`
 * https://github.com/spring-projects/spring-security/issues/11939[gh-11939] - Remove deprecated `antMatchers`, `mvcMatchers`, `regexMatchers` helper methods from Java Configuration.
 Instead, use `requestMatchers` or `HttpSecurity#securityMatchers`.
+* https://github.com/spring-projects/spring-security/issues/11985[gh-11985] - Remove deprecated constructors in `Argon2PasswordEncoder`, `SCryptPasswordEncoder` and `Pbkdf2PasswordEncoder`.