Revert issue_370251_remove_namespace_agg_scheduler_lease

What does this MR do and why?

Based on the findings in the Rollout Issue, we've decided to remove the remove_namespace_agg_scheduler_lease FF and decided to reduce the lease duration instead of removing it totally. The FF is already disabled on production

ref: https://gitlab.com/gitlab-org/gitlab/-/issues/370251

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