Remove WorkItemUpdateWidgets mutation

WorkItemUpdateWidgets was originally introduced as the result of a conversation about making mutation smaller and more specific during the first work item widgets implementation.

Now we are only using a more general mutation WorkItemUpdate to update all widgets and other fields of a work item.

frontend still uses the WorkItemUpdateWidgets mutation to update descriptions. We should use the more generic workItemUpdate mutation and on the next milestone remove the mutation from the API.

The mutation is still behind a feature flag and also marked as experimental, so we shouldn't follow the regular deprecation process to remove it, no external client should be using it.