Enable work_item_features_field by default

What does this MR do and why?

All three widget-splitting MRs are merged and live in production, so the work_item_features_field flag can move from wip to beta and default on for every instance. wip flags can't be default-enabled, hence the type change.

This also drops the suite-wide stub_feature_flags(work_item_features_field: false) from spec_helper.rb, so RSpec exercises the features path rather than the widgets one.

Draft on purpose: holding this for a week so any breakage from the merged splits can surface in production first.

References

• #587972 (closed)
• gitlab-org#20609
• Blocked by !253547 (merged), which adds the missing unauthenticated complexity guard for the work item detail queries.
• Blocked by !259817 (merged), which fixes the features path bugs this MR's pipeline surfaced on boards and epic linked items.

How to set up and validate locally

1. Make sure the flag is unset locally, so the new default applies:

   Feature.remove(:work_item_features_field)

2. Open a work item and confirm it renders, then check the network tab shows namespaceWorkItem requesting features rather than widgets.

3. Sign out and open a public work item, and confirm it renders instead of Work Item not found.

MR acceptance checklist

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