Migrate to Artifact Registry from Container Registry

[NimJay]

• Today, Online Boutique uses Google Cloud's Container Registry for the container images that it published.
• For instance, gcr.io/google-samples/emailservice:v0.5.0 is hosted on Container Registry.
• However, Artifact Registry is the recommended way to host container images on Google Cloud — not Container Registry.
• Relevant documentation: Transition from Container Registry.
• We should migrate to Artifact Registry.

[NimJay]

Update (to cool down our teams' OoSLO list): This work was originally planned for 2023 Q4, but it could be delayed since our team's priorities have changed significantly.

[minherz]

Priority is decreased because the required migration is not urgent.

[muncus]

This continues to be low-priority work.