Fallback to ILIKE search for group work items

What does this MR do and why?

For us to allow full-text search on group-level work items, we need to repartition the issue_search_data table which requires multiple milestones for the backfilling and finalizing of background migrations.

To allow us to develop and test out features (like work item search for the hierarchy parent dropdown), we fallback to ILIKE searching for now.

We don't have a trigram index for this but when searching within groups we intentionally avoid that anyway by using a CTE.

This is behind the namespace_level_work_items feature flag which is wip and under active development.

Sample query: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/25770/commands/81250

MR acceptance checklist

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How to set up and validate locally

1. Enable namespace_level_work_items
2. Visit /groups/<group>/-/work_items to create a group-level work item (Epic) (Note: the list here is not fully functional yet)
3. Visit /groups/<group>/-/issues (this includes group-level work items) and test out the text search

Related to #393126 (closed)