Move Docker images from Container Registry to Artifact Registry

[lvaylet]

cloudbuild.yaml currently pushes Docker images to Google Cloud Registry (GCR). However, as highlighted in the Google Cloud documentation:

Artifact Registry is the recommended service for container image storage and management on Google Cloud. As a fully-managed service with support for both container images and non-container artifacts, Artifact Registry extends the capabilities of Container Registry.

Transition from Cloud Registry to Artifact Registry by updating cloudbuild.yaml (and everything related in the build/tag/deploy process, like docs, Terraform scripts and Makefile targets).

[lvaylet]

For a complete list of files to update in this repo (including docs and tests), do a search on gcr.io:

Then https://github.com/terraform-google-modules/terraform-google-slo probably needs some updates too, for example in modules/slo-generator/main.tf.