roachprod: GCE cost estimation API will be discontinued as of June 15, 2025

GCE cost estimation API is currently used by roachprod [1], behind a feature flag. It will be discontinued as of June 15, 2025. Also note that support for AWS and Azure are both open items [2].

Email from GCP,

> We’re writing to inform you that the public preview [Cost Estimation API](https://cloud.google.com/billing/docs/reference/cost-estimation/rest) will be discontinued on June 15, 2025. This Application Programming Interface (API) was deprecated in May 2024.

> To continue getting Google Cloud cost estimates, you must migrate to one of the following options:
> User Interface (UI): Use the updated [Google Cloud Pricing Calculator](https://cloud.google.com/products/calculator?hl=en), which currently shows contractual prices for all services.
API: Use the [Cloud Billing Pricing API](https://cloud.google.com/billing/docs/reference/pricing-api/rest) to programmatically get contractual prices.
Please note: This API’s interface differs from the deprecated Cost Estimation API.

[1] https://github.com/cockroachdb/cockroach/pull/98955
[2] https://github.com/cockroachdb/cockroach/issues/106880