Update the GCF resource to reflect transition from Container Registry to Artifact Registry

[Ectelion]

The general transition from Container Registry is outlined in this public doc. The GCF-specific API transition has already completed. The field docker_registry was already marked as optional and computed -- no need for any explicit changes to the defaulting logic here, we only update the docs and comments to reflect the new auto-defaulting.

Release Note Template for Downstream PRs (will be copied)

cloudfunctions: Updated docs for Cloud Function's `docker_registry` field to reflect the change of default from Container Registry to Artifact Registry.

[github-actions[bot]]

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[rileykarson]

/gcbrun

[rileykarson]

Running tests to make sure there are no surprises, but lgtm assuming there's none

[Ectelion]

@rileykarson thanks!

[Ectelion]

/gcbrun