chore: centralize ingress dependency versions

[danehans]

Description

Define Gateway API, agentgateway, and Envoy Gateway versions in one machine-readable source and add an automated check that keeps documentation and test setup synchronized.

Context

Ingress dependency versions are currently repeated across docs/kubernetes/ingress.mdx, tasks/scripts/agentgateway-k8s-setup.sh, and e2e/with-kube-gateway.sh. These values can drift; for example, the documented Envoy Gateway version and the E2E default are currently different. This was identified while reviewing #3714.

Definition of Done

• Gateway API, agentgateway, and Envoy Gateway defaults have one machine-readable source of truth.
• Kubernetes setup and E2E scripts consume those defaults while preserving supported environment-variable overrides.
• Documentation uses or is validated against the same defaults.
• CI fails when documented and tested ingress dependency versions drift.
• The version update workflow is documented.