PR needing review page should use create date not update date

[LonMcGregor]

Only using the most recent update date gives a false impression of our review status.

This is a problem if a trainee has an old review but they toggled the "needs review" label in an attempt to get our attention, as I saw happen recently.

We should always use the oldest date here.

Thoughts?

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[illicitonion]

I've been modelling this as "time spent waiting" rather than "time since it was created"...

Would a best of both worlds be to be checking label transitions? i.e. "When was Needs Review last added when before it either didn't have any relevant label, or had the label Reviewed"?

I don't mind making this change as-is if it's simpler and useful, but I suspect "I opened a PR a long time ago but only just acted on feedback and re-requested review" is a more common occurrence than "I toggled the Needs Review label because I got frustrated"