Increase work_item_children_spec query limit

What does this MR do and why?

Fixes broken master pipeline: Monday 2024-08-12 21:58 UTC - `gitlab-org/gitla... (gitlab-org/quality/engineering-productivity/master-broken-incidents#7980 - closed)

issue: [Test] spec/features/projects/work_items/work_i... (#474318 - closed)

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