Move istio to base component for Reliability tenant

What

The change moves Istio from the overlays/gke/gitlab-staging-1/us-east1/gstg-gitlab-gke/istio directory to the components/base directory. This change will apply Istio to the Reliability tenant.

This will ensure the flux-components-istio is pulled and installed ahead of any overlays.

Why

We are experiencing a race condition for the ConfigMaps generated by the overlays, since the namespace hasn't been created ahead of them.

Related Issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24157