Fix date save for Epic to reflect on UI immediately after save

What does this MR do?

Fixes a bug where saving Epic start/finish dates on UI is not reflected immediately after save.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder

Closes #10543 (closed)