Work item features - Rollout work_item_features_field FF

Phase 4-6: Rollout and cleanup of the work_item_features_field feature flag.

Progressively enable the feature flag for internal users (GitLab team members), monitor for issues, and eventually remove the old widgets code paths once confident in the new implementation. This includes turning on the flag, validating stability, and cleaning up legacy code.

Requirements

1. Make sure to progressively update each feature flag disabled in feature specs. Right now it's disabled globally in spec/spec_helper.rb
2. Make sure to remove the extended query text size from the specs added in !222922 (d4f2730d)
3. remove the QA spec stubbing of the flag in !222922 (4d521898)

As we iterate, we can either manually enable them, or remove the stub and make sure all pass