readme: images are now mirrored to GCR and then to Harbor

[maelvls]

The idea is to rationalise how images are replicated, especially because we want to start (later) adding FIPS images to Venafi Kubernetes Agent. The change has been made in https://gitlab.com/venafi/vaas/delivery/harbor/-/merge_requests/48.

Before:

v1.1.0 (Git tag in the jetstack-secure repo)
 └─ quay.io/jetstack/venafi-agent:v1.1.0 (GitHub Actions in the jetstack-secure repo)
     ├── registry.venafi.cloud/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)
     ├── private-registry.venafi.cloud/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)
     └── private-registry.venafi.eu/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)

After:

v1.1.0 (Git tag in the jetstack-secure repo)
 └── quay.io/jetstack/venafi-agent:v1.1.0 (GitHub Actions in the jetstack-secure repo)
     ├── us.gcr.io/jetstack-secure-enterprise/venafi-agent (Enterprise Builds's GitHub Actions)
     └── eu.gcr.io/jetstack-secure-enterprise/venafi-agent (Enterprise Builds's GitHub Actions)
         ├── registry.venafi.cloud/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)
         ├── private-registry.venafi.cloud/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)
         └── private-registry.venafi.eu/venafi-agent/venafi-agent:v1.1.0 (Harbor Replication)

Refs:

• https://github.com/jetstack/enterprise-builds/pull/224
• https://gitlab.com/venafi/vaas/delivery/harbor/-/merge_requests/48

[inteon]

Thanks for the docs update!