Change feature flag and licensed feature name for custom status design doc

Why is this change being made?

Rename status feature flag and licensed feature (gitlab-org/gitlab!184157 - merged) changes the feature flag and licensed feature for custom status:

1. Feature flag: work_item_status --> work_item_status_feature_flag
2. Licensed feature: work_item_custom_status --> work_item_status

This MR adds the change to the design doc.

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Commits

• Change feature flag and licensed feature name for custom status design doc

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