Migration guide for CAS support removal

Issue gh-12163

Author: marcusdacoregio

docs/modules/ROOT/nav.adoc

@@ -11,6 +11,7 @@
 *** xref:migration/servlet/authorization.adoc[Authorization]
 *** xref:migration/servlet/oauth2.adoc[OAuth]
 *** xref:migration/servlet/saml2.adoc[SAML]
+*** xref:migration/servlet/cas.adoc[CAS]
 ** xref:migration/reactive.adoc[Reactive Migrations]
 * xref:getting-spring-security.adoc[Getting Spring Security]
 * xref:features/index.adoc[Features]

docs/modules/ROOT/pages/migration/servlet/cas.adoc

@@ -0,0 +1,10 @@
+= CAS Migrations
+
+The following steps relate to changes around CAS support.
+
+== Remove CAS support
+
+In Spring Security 6.0, the CAS support https://github.com/spring-projects/spring-security/issues/10441[has been removed], see the linked issue for more information.
+There is no direct replacement for it, however, it is possible to https://apereo.github.io/cas/6.6.x/authentication/OAuth-Authentication.html[configure your CAS server to act as an OAuth 2.0 Authentication Provider] and use the xref::servlet/oauth2/index.adoc[OAuth 2.0 support in Spring Security].
+
+The support might be re-added in future versions of Spring Security, subscribe to https://github.com/spring-projects/spring-security/issues/11674[this issue] to receive updates.