Apply sizing recommendations in GCP

In https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5151#note_141985734, we investigated a cost optimization tool for our GCP cloud spend. We learned that it's difficult to beat Google's own recommendations in the GCP billing panel.

The only recommendation that I'm aware of in the billing panel is a recommendation to resize our instances:

We should evaluate their recommendations and consider sizing down to achieve some/all of the $17k/month in savings.