Update What's New section

Adds the SAML 2.0 Single Logout Support and the new Saml2AuthenticationRequestRepository

Author: marcusdacoregio

docs/manual/src/docs/asciidoc/_includes/about/whats-new.adoc

@@ -9,3 +9,8 @@ Below are the highlights of the release.
 * Configuration
 
 ** Introduced https://github.com/spring-projects/spring-security/pull/9630[`AuthorizationManager`] for method security
+
+* SAML 2.0 Service Provider
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+** Added https://github.com/spring-projects/spring-security/pull/9483[SAML 2.0 Single Logout Support]
+** Added https://github.com/spring-projects/spring-security/pull/10060[Saml2AuthenticationRequestRepository]