Cleanup legacy VPC resources

• Clean up all values in gitlab-ci variables vpc_peering_networks 👉 https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/9869
• Clean up CIDR rules in the ci-gateway-allow-runners in gprd.
• Clean up Runbooks CIDR list (+add one missing CIDR for small-6?) 👉 gitlab-com/runbooks!8307 (merged).
• Clean up gprd variables: ci-gateway-network-peers 👉 https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/9870.