Sort epics by created_at, updated_at in GraphQL

We had a report from @agroleau that he could no longer access these facets of epic search when the frontend search widget was moved over from REST to Vue using GraphQL. Other customers may be affected.

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We currently offer the ability to sort by created_at and updated_at in REST, but not in GraphQL.

This creates a degradation in user experience when switching to the search component implemented in Vue as the ability to sort by last created_at and updated_at is not there. This was noticed by @agroleau.

We should introduce it in GraphQL so that parity is maintained when the new experience is rolled out to external users.

Here are the available GraphQL sort criteria 👉 ee/app/graphql/types/epic_sort_enum.rb#L15

Here are the available REST sort criteria 👉 ee/lib/api/epics.rb#L35