Zoekt: Only enable global search when nodes are online

What does this MR do and why?

This is related to Zoekt attempts to search offline nodes (#543145 - closed)

I've discovered that Search::Zoekt::RoutingService returns offline nodes

Steps to reproduce

1. Stop Zoekt processes
2. Wait until the nodes are offline
3. Execute

   Search::Zoekt::RoutingService.execute(Project.where(id: [7]))

4. It will return offline nodes

This MR fixes this problem by ensuring that we only return online nodes.

Query plans

Before

• https://postgres.ai/console/gitlab/gitlab-production-main/sessions/40747/commands/125188

def before
  ActiveRecord::Base.logger = Logger.new $stdout

  projects = Project.inside_path('gitlab-org')

  projects.without_order.joins(zoekt_repositories: { zoekt_index: { replica: :zoekt_enabled_namespace } })
    .merge(Search::Zoekt::EnabledNamespace.search_enabled).merge(Search::Zoekt::Replica.ready)
    .pluck(:id, :zoekt_node_id)

  true
end

After

• https://postgres.ai/console/gitlab/gitlab-production-main/sessions/40747/commands/125189

def after
  ActiveRecord::Base.logger = Logger.new $stdout

  projects = Project.inside_path('gitlab-org')

  projects.without_order
    .joins(zoekt_repositories: { zoekt_index: { replica: :zoekt_enabled_namespace, node: nil } })
    .merge(Search::Zoekt::EnabledNamespace.search_enabled)
    .merge(Search::Zoekt::Replica.ready)
    .merge(Search::Zoekt::Node.online)
    .pluck(:id, :zoekt_node_id)

  true
end

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #550604 (closed)