[19.3] Raise stable pulp job timeout to 12h

Lachlan Grantrequested to merge
timeout-12h-19-3-stable into 19-3-stable

Backports !7363 (merged) to 19-3-stable, to unblock the stuck v19.3 patch pipeline - its stable pulp: [distro] jobs have been timing out at 4h. Temporary measure pending the real fix in gitlab-com/gl-infra/production-engineering#29705.

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