Moved epic assignees to work_items_beta FF

Donald Cookrequested to merge
dc/wi-epics-assignees into master

What does this MR do and why?

devopsplan decided that we can turn on assignees on epic work items for internal users. We will not want this to go GA with the rest of the epics to work item work. The easiest way is to switch the feature flag to the one that we are using to "promote" features within the work items framework.

Once this is merged, I will merge gitlab-com/content-sites/handbook!7907 (merged) which documents the feature flag usage here.

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