Design a manual process for verifying GCP backups

[yuvipanda]

The outcome of #4536 was that there's no easy automated way for us to monitor GCP disaster recovery backups.

Let's design a manual process that we can do every month instead.

The outcome of this should be:

Definition of done

• A document that any engineer can follow to check if backups are being performed correctly, and old backups are being deleted correctly.
• This process should not take more than a total of 30min for the engineer to perform.

Design options

There are three ways to do this:

1. Document how you would look at this in the GUI
2. Document what set of gcloud commandline commands should be run and how to verify them
3. Add a command to deployer to do this

Whoever is doing this task can decide which of these three approaches to take. Take whatever is the easiest for you to implement, so we can mark move forward in this EPIC.