Handle fixed dates seperately from selected dates in Epics

What does this MR do?

Fixes a bug where changing date type from Milestones to Fixed in Epics would affect existing Fixed date value (by duplicating milestone date into fixed). This feature was introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6658.

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 #7437 (closed)