Add ACL Migration Steps

docs/modules/ROOT/pages/migration-7/acl.adoc

@@ -0,0 +1,7 @@
+= ACL
+
+== Favor `AclPermissionEvaluator`
+
+`AclEntryVoter`, `AclEntryAfterInvocationProvider`, and `AclPermissionEvaluator` provide the same service, plugged into different Spring Security APIs. Now that `AccessDecisionVoter` and `AfterInvocationProvider` are both deprecated, the corresponding ACL plugins are obsolete.
+
+As such, begin using `AclPermissionEvaluator` before updating to Spring Security 7.