Make LOST_DURATION_THRESHOLD configurable in admin setting

Background

Currently, the nodes are marked as lost when they are offline for more than 12 hours This is a hardcoded value and gives less flexibility for the self-managed customer.

Proposal

Move this constant to the admin setting and allow admins to select this value.

Note: Add the setting attribute here as well: https://gitlab.com/gitlab-org/gitlab/-/blob/5a1a34b19ac483a82f430ee493acf99b194405fa/ee/app/models/search/zoekt/settings.rb