CI: Measure TestGrid Flakiness workflow fails due to DNS resolution error for testgrid-data.k8s.io

## Problem

The [Measure TestGrid Flakiness](https://github.com/etcd-io/etcd/blob/main/.github/workflows/measure-testgrid-flakiness.yaml) scheduled workflow has been failing consistently with a DNS resolution error.

## Error

## Example Failed Runs

- https://github.com/etcd-io/etcd/actions/runs/26989330942/job/79645888864 (2026-06-05)

## Context

This appears to be an external infrastructure issue with the Kubernetes TestGrid data endpoint. The domain cannot be resolved from GitHub Actions runners.

The script was originally added in #17773 by @siyuanfoundation, but that issue was auto-closed by stale bot and the author is no longer active in the project.

## Questions

1. Is the TestGrid endpoint URL still correct, or has it changed?
2. Should this workflow be updated, disabled, or removed if the external dependency is no longer available?

cc @serathius @jmhbnz

Start with .github/workflows/measure-testgrid-flakiness.yaml and the failed GitHub Actions run linked in the issue. Check whether testgrid-data.k8s.io resolves and whether the TestGrid endpoint is still valid; determine whether the workflow should be updated, disabled, or removed, then verify the scheduled workflow no longer fails.

Written by the indexing model from the issue text.