Corrective action: CI Gateway in us-east1-d experiencing connectivity issues

Summary

Update ci-runners networking documentation and note that when adding/updating/removing CIDRs allocations for ephemeral runner VMs we also need to update the ci-gateway ILB firewall.

We also discussed adding a comment in Terraform with a pointer to the firewall rules.

Related Incident(s)

Originating issue(s): production#7084 (closed)

Desired Outcome/Acceptance Criteria

Associated Services

Corrective Action Issue Checklist

• Link the incident(s) this corrective action arose out of
• Give context for what problem this corrective action is trying to prevent from re-occurring
• Assign a severity label (this is the highest sev of related incidents, defaults to 'severity::4')
• Assign a priority (this will default to 'priority::4')