Validate parent and child depth and cyclic references

What does this MR do and why?

Adds hierarchy-sepcific restrictions:

• checks for cyclic references
• checks for maxium_depth

Related to #378801 (closed)

DB queries:

Note: we don't support creating multi-level hierarchy yet, queries below are for a random issue task in gitlab/gitlab-org project but there are no ancestors/descendants for it.

To get ancestors/descendants, this MR adds an object hierarchy (lib/gitlab/work_items/work_item_hierarchy.rb, heavily inspired by lib/gitlab/ci/pipeline_object_hierarchy.rb).

• work_item.ancestors: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/13634/commands/47880
• work_item.same_type_descendants_depth: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/13634/commands/47881
• work_item.same_type_descendants_depth : https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/13634/commands/47882

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How to set up and validate locally

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