Fix `fetchGroupEpics` exported as Vuex action but doesn't follow API

What does this MR do?

This MR fixes an issue raised in a previous MR review.

fetchGroupEpics is an extracted function used by a couple of functions in store/actions.js. It was exported which implies it can be dispatched as a Vuex action, but this is not the case. This MR fixes this by making fetchGroupEpics a "private" function within its actions.js file

Issue: #212984 (closed)

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Does this MR meet the acceptance criteria?

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Closes #212984 (closed)