Update links to 5.8 migration guide

Closes gh-13156

docs/modules/ROOT/pages/migration/index.adoc

@@ -1,10 +1,11 @@
 [[migration]]
 = Migrating to 6.0
+:primary-site-url: https://docs.spring.io/spring-security/reference
 
 The Spring Security team has prepared the 5.8 release to simplify upgrading to Spring Security 6.0.
 Use 5.8 and
 ifdef::spring-security-version[]
-xref:5.8.2@migration/index.adoc[its preparation steps]
+{primary-site-url}/5.8/migration/index.html[its preparation steps]
 endif::[]
 ifndef::spring-security-version[]
 its preparation steps

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

@@ -1,8 +1,9 @@
 = Exploit Protection Migrations
+:primary-site-url: https://docs.spring.io/spring-security/reference
 
 The 5.8 migration guide contains several steps for
 ifdef::spring-security-version[]
-xref:5.8.2@migration/servlet/exploits.adoc[exploit protection migrations] when updating to 6.0.
+{primary-site-url}/5.8/migration/servlet/exploits.html[exploit protection migrations] when updating to 6.0.
 endif::[]
 ifndef::spring-security-version[]
 exploit protection migrations when updating to 6.0.