Resolve "Cannot update Epic is_fixed columns alone in API"

What does this MR do?

Allow updating start_date_is_fixed or due_date_is_fixed fields alone in Epic update API endpoint.

Breaking change is determined to be okay, and some level of backward compatibility has been implemented so existing calls will not err.

Update Epic create API doc to reflect the changes related to new date fields.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7568

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7568 (closed)