Fix EpicBaseEntity human_readable_timestamp

What does this MR do?

Fixes regression introduced by a refactor that changed #remaining_day_in_words helper method to accept a due_date and an optional start_date instead of the entity itself.

This problem was missed because the issue_serializer_spec.rb (the only place where EpicBaseEntity is used is in the issue serializer for the sidebar) creates an epic without either due dates or start dates, so I've changed that to avoid another regression.

What are the relevant issue numbers?

#9106 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
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• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
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• Security reports checked/validated by reviewer

Closes #9106 (closed)