docs: Update authz-custom to new OPA docs link (#16501)

We have moved some pages in the OPA site and this content has a new URL.

Author: charlieegan3

content/en/docs/tasks/security/authorization/authz-custom/index.md

@@ -8,7 +8,7 @@ test: yes
 ---
 
 This task shows you how to set up an Istio authorization policy using a new value for the [action field](/docs/reference/config/security/authorization-policy/#AuthorizationPolicy-Action), `CUSTOM`,
-to delegate the access control to an external authorization system. This can be used to integrate with [OPA authorization](https://www.openpolicyagent.org/docs/latest/envoy-introduction/),
+to delegate the access control to an external authorization system. This can be used to integrate with [OPA authorization](https://www.openpolicyagent.org/docs/envoy),
 [`oauth2-proxy`](https://github.com/oauth2-proxy/oauth2-proxy), your own custom external authorization server and more.
 
 ## Before you begin