Finish phasing out Kubernetes channels

[themightychris]

For #3200, completed through PRs #3205 and #3214, we simplified the Kubernetes deployment process and eliminated the concept of "channels" which encapsulated different sets of services but were all deployed to the same cluster.

One lingering artifact of the previous setup is that services deployed to the production cluster are still segmented between prod.yaml and test.yaml under ci/channels. prod.yaml is now hardcoded into the deployment workflows and is the only "channel" that matters now, but test.yaml still contains definitions of several test instances that are deployed to the same cluster but currently unmanaged by the deployment process. These test instances are useful when deploying new versions of Metabase or the GTFS RT archiver

Work to be done

• Merge -test instances into prod.yaml and finish deprecating the concept of channels
• Reconcile the current state of the cluster against the new declared state
• Test deploying a change to one of the test instances and verify that the PR workflow is smooth and correct