key_retired should be key_discarded

[rmarx]

Apparently, what was once (colloquially) known as key "retire" is now called key "discard" in RFC 9001.

As such, we should update the event definition's name (and probably add some more information in the qlog spec as well, since it's a bit barebones atm). I'm also not sure anymore what the difference is in the triggers with "tls" vs "local/remote update"...

cc @kosekmi