Update previous epic milestone dates when issue switches epic

What does this MR do?

When an issue switches epic, the previous epic is not updated for its milestone dates. This MR ensures both the old and new epic are updated.

What are the relevant issue numbers?

#7869 (closed)

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 #7869 (closed)