EndpointSlice API GA Doc Updates for 1.21
* Deprecate `endpointslices.discovery.k8s.io/v1beta1` * Removes `content/en/docs/tasks/administer-cluster/enabling-endpointslices.md` and redirects requests to that page to `content/en/docs/concepts/services-networking/endpoint-slices.md`
This commit fixes errors which are shown by linkchecker.py on en/docs/*.