Refactor how expired namespaces are removed from index

What does this MR do and why?

Related to #439769 (closed)

This MR adjusts what qualifies as an "expired" namespace for removal from advanced search. The previous logic was missing removing trials that are null and as a result there are a good number of namespaces that need to be removed from the index.

Due to the large number (see internal comment, I've added a limit of 1000 namespaces that can be removed in one run. The cronworker runs once per day and I think it's ok to remove things slowly (they have been in the index this long so a slow removal will be safer)

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Screenshots or screen recordings

N/A this is changes to a cron job

How to set up and validate locally

1. create 2 ElasticsearchIndexedNamespace records with default plan subscriptions

    plan = Plan.default
    Group.last(2).each do |g| 
      GitlabSubscription.create(namespace_id: g.id, hosted_plan_id: plan.id)
      ElasticsearchIndexedNamespace.create(namespace_id: g.id)
    end

2. create 2 ElasticsearchIndexedNamespace records with bronze plan subscriptions

    plan = Plan.find_by_name('bronze')
    groups = Group.first(2)
    # one with no expiration
    GitlabSubscription.create(namespace_id: groups.first.id, hosted_plan_id: plan.id)
    ElasticsearchIndexedNamespace.create(namespace_id: groups.first.id)
    # one with an expired subscription expiration
    GitlabSubscription.create(namespace_id: groups.second.id, hosted_plan_id: plan.id, start_date: 1.months.ago, end_date: 2.months.ago)
    ElasticsearchIndexedNamespace.create(namespace_id: groups.second.id)

3. verify that there are 4 records in ElasticsearchIndexedNamespace table

   ElasticsearchIndexedNamespace.count

4. run the destroy command

   ElasticsearchIndexedNamespace.destroy_namespaces_with_expired_subscriptions!

5. verify that there is 1 record in ElasticsearchIndexedNamespace table

   ElasticsearchIndexedNamespace.count