Fix creating epics with dates from api

What does this MR do?

When we set start_date_fixed (or due_date) in the api, it goes to the create service where the dates are not handled correctly. start_date/end_date fields need to be filled so that everything works as expected.

I added the method to handling dates to the Epics::CreateService. Additionally I overrode assign_attributes method, because the method caused assigning non-permitted params in the service (which made it more difficult to debug among other things).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #13360 (closed)