Add Link to 6.0 Migration Guide

Issue gh-12093

Author: jzheaux

docs/modules/ROOT/pages/migration.adoc

@@ -2,7 +2,14 @@
 = Migrating to 6.0
 
 The Spring Security team has prepared the 5.8 release to simplify upgrading to Spring Security 6.0.
-Use 5.8 and the steps below to minimize changes when updating to 6.0.
+Use 5.8 and the steps below to minimize changes when
+ifdef::spring-security-version[]
+xref:6.0.0@migration.adoc[updating to 6.0]
+endif::[]
+ifndef::spring-security-version[]
+updating to 6.0
+endif::[]
+.
 
 == Servlet