Add 'created_at' and 'updated_at' to epic API

What does this MR do?

Allows the created_at and updated_at fields to be set in create and update in API requests respectively (to users with owner privileges). Brings the Epic API into closer parity with the issues api.

re #255309 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done