Refactor work item cache update to mutate update hook

What does this MR do and why?

This MR refactors work item add_hierarchy_child.mutation.graphql/remove_hierarchy_child.mutation.graphql client mutations to mutate update hooks instead, which makes better sense as we want to update the cache exactly when we perform the mutation to add/remove a child work item

Screenshots or screen recordings

No changes

How to set up and validate locally

1. On an issue, test adding/removing tasks in the tasks widget
2. On an issue, test converting checklist items in the description to tasks

MR acceptance checklist

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