Update pipeline freshness windown to 16h

What does this MR do and why?

Ref: https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/4035

• This MR extends the merge train pipeline freshness window from 8 hours to 16 hours.
• This change is part of our ongoing effort to reduce RSpec pipeline costs by minimizing unnecessary tier-3 pipeline reruns.
• 16 hours should be sufficient to accommodate most asynchronous timezone handoffs between globally distributed teams, allowing approved MRs to merge without forcing fresh pipeline runs during normal collaboration delays.

Related triage-ops MR - gitlab-org/quality/triage-ops!3745 (merged)

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