Document how to manually rerun periodic Prow jobs

[wallrj-cyberark]

Motivation

There was no documentation on how to manually retrigger periodic Prow jobs.
This came up during the cert-manager 1.21 release preparation: after merging
a .trivyignore update (cert-manager/cert-manager#8983) to fix trivy scan
failures caused by GO-2026-5932, we needed to retrigger the periodic trivy
jobs rather than waiting for their next cron cycle.

Changes

Adds make list-periodics and make rerun-failed-periodics targets backed
by a prow/periodic-jobs.sh script that queries the Prow API with jq:

• make list-periodics BRANCH=release-1.21 — lists the latest run of each
  periodic job matching the search term, showing state, start time, and
  ProwJob ID.
• make rerun-failed-periodics BRANCH=release-1.21 — reruns every
  non-success periodic job for the given branch in one go.

Updates prow/README.md with a "Rerunning periodic jobs" section covering:

• Why you might need to do this (e.g. pre-release reassurance after
  dependency updates)
• Prerequisites (GCP IAM role, kubectl cluster credentials)
• Usage of the make targets

Testing

$ make list-periodics BRANCH=release-1.21
ci-cert-manager-release-1.21-e2e-v1-33: pending (started: 2026-07-10T09:06:40Z, id: c1b96b54-...)
ci-cert-manager-release-1.21-e2e-v1-33-feature-gates-disabled: success (started: 2026-07-09T10:35:40Z, id: 7a8410fd-...)
...

$ make rerun-failed-periodics BRANCH=release-1.21
Rerunning: ci-cert-manager-release-1.21-e2e-v1-33
# (kubectl create would follow with cluster access)

$ make list-periodics
Makefile:51: *** BRANCH is required, e.g. make list-periodics BRANCH=release-1.21.  Stop.

[cert-manager-prow]

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