UI: Stage Timestamp shows freight creation date, not freight promotion date.

[dhpup]

Checklist

• [X] I've searched the issue queue to verify this is not a duplicate feature request.
• [X] I've pasted the output of kargo version, if applicable.
• [X] I've pasted logs, if applicable.

Proposed Feature

Right now the Timestamp shows the freight creation date, would it make more sense to show when the promotion occured?

This came as I tested multiple Stages with auto-promote enabled. But left a Stage lingering in a progressing state for a long while (verification). Once the verification was complete, it showed the next Stage with a timestamp that wasn't recent which was what I expected.

Motivation

Clarity in the UI for end users.

Suggested Implementation

Should it show promotion date instead?

[krancour]

@rbreeze this needs a small back-end change as well. Up to it?

We need to start recording the time that a Stage's current promotion transitions to become its last promotion.

I propose this to be a new CompletionTime field in the PromotionInfo struct (which I think could stand to be renamed as PromotionReference while we are at it, as that is more consistent with FreightReference).

cc @hiddeco for a sanity check.