What type of PR is this?

/kind feature

What this PR does / why we need it:

We are caching the release markers at dl.k8s.io. Details are at https://kubernetes.slack.com/archives/CJH2GBF7Y/p1770744489128559

This PR does a few things:

1. Adds Surrogate-Key: [prod|ci]-markers headers when uploading markers, which allows Fastly to track these files with these headers and purge them together.
2. We look for the FASTLY_SERVICE_NAME env variable. If it's set, we attempt to purge the specific keys mentioned, and it should take effect immediately.
3. I amended the marker verification to now check every 20 seconds for 3 minutes and fail if the markers don't match. In theory, it should be instant, but this should help keep the pipeline robust.
4. I already added a purge-only token for Fastly to the k8s-release project.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE