Update release branch jobs creation documentation

[xmudrii]

We should update release branch jobs creation documentation which is part of the Release Manager handbook to reflect notes from kubernetes/test-infra#29387

I'll provide more details about this if needed.

We should also coordinate this with kubernetes/test-infra#29390

[xmudrii]

/sig release
/priority important-soon
/milestone v1.28

[k8s-triage-robot]

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• After 90d of inactivity, lifecycle/stale is applied
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/lifecycle stale

[xmudrii]

/remove-lifecycle stale

[dims]

/milestone v1.31