[FEATURE] Remove GCP guidance from CRD, proto and API documentation #3674

CRD descriptions, proto comments and the API guide use GCS-only examples and GKE-specific guidance. Make them provider-neutral. Only descriptions change; schemas stay the same.

Tracks "Remove GCP guidance from CRD and API documentation" in the Provider-Neutral Core workstream of the Extensible Install Roadmap. A docs purge was mentioned as a follow-up in #3505.

Current state (0b635b80)

• api/infra/k8s/ate/v1/sandboxconfig_types.go, and the generated ate.dev_sandboxconfigs.yaml;
• api/objectstore/v1/objectstore.proto comments;
• docs/reference/api-guide.md: gs:// examples, GCS fallback text, the ComputeClass toleration sentence.

Required changes

• Replace gs:// examples with scheme-neutral or HTTPS examples, and remove GKE-specific comments.
• Regenerate the CRDs and protos.
• The ComputeClass sentence goes with Move atelet's GKE ComputeClass toleration into the GKE install overlay #2760. The gVisor URL examples follow [FEATURE] atelet fetches public sandbox assets over HTTPS #3672.

Dependencies: none for most of it. The asset URL examples follow #3672 step 1.

Breaking change: none.

Enforcement: the generated CRD schemas are unchanged apart from description fields.

Parent Issue: #3161