[Atlas] Specify that GCP clusters that are M10 or M20 also run on burstable performance infrastructure

Dedicated Clusters for Low-Traffic Applications - On the page: https://docs.atlas.mongodb.com/cluster-tier/#dedicated-clusters-for-low-traffic-applications

It is stated that: M10 and M20 cluster tiers on AWS and Azure run on burstable performance infrastructure.

When in fact, it is the case for GCP too.