Reduce maximum depth for work item subepics to 7

What does this MR do and why?

As a solution for Creation of child epics beyond 7 levels is failing (#472097 - closed) it was agreed to reduce the maximum of nested levels allowed for work item epics from 9 to 7.

This MR includes a DB migration to update the maximum_depth value for the work_item_hierarchy_restrictions record corresponding to the epic-epic relationship.

Database

Migration UP

bundle exec rails db:migrate:up:main VERSION=20240712093230

main: == [advisory_lock_connection] object_id: 130120, pg_backend_pid: 92617
main: == 20240712093230 UpdateHierarchyRestrictionSubepicsMaximumDepth: migrating ===
main: == 20240712093230 UpdateHierarchyRestrictionSubepicsMaximumDepth: migrated (0.0352s)

main: == [advisory_lock_connection] object_id: 130120, pg_backend_pid: 92617

Plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29867/commands/92800

Migration DOWN

bundle exec rails db:migrate:down:main VERSION=20240712093230

main: == [advisory_lock_connection] object_id: 130180, pg_backend_pid: 92067
main: == 20240712093230 UpdateHierarchyRestrictionSubepicsMaximumDepth: reverting ===
main: == 20240712093230 UpdateHierarchyRestrictionSubepicsMaximumDepth: reverted (0.0325s)

main: == [advisory_lock_connection] object_id: 130180, pg_backend_pid: 92067

Plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29867/commands/92803

MR acceptance checklist

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Screenshots or screen recordings

max_depth_reached_error

How to set up and validate locally

1. Enable feature flag Feature.enable(:work_item_epics)
2. Create an epic in any group and open it using the work item view, e.g. http://127.0.0.1:3000/groups/GROUP_PATH/-/work_items/EPIC_IID
3. Using the Child items add new epics until having a total of 7 nested levels
4. Verify that adding a child to the deepest epic returns cannot be added: reached maximum depth. For this it's necessary to check the Network Monitor tab in your browser because errors are not shown in the UI due to a bug.