Use last_edited_at instead of updated_at for displaying issuable last change

Why was this MR needed?

In UI bellow description we display who and when updated the item. But last_edited_by (used for 'who') is mistakenly combined with updated_at (when), last_edited_at should be used instead.

We may also consider renaming response attributes in #realtime_changes and other methods from updated_* to last_edited_* to not collide with model's updated_at attribute. This will require also frontend changes though and can be done separately.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

EE version of this patch:

• gitlab-ee!4042

What are the relevant issue numbers?

• gitlab-ee#4416

Closes #41247 (closed)