hkp: return correct creation time

The creation time reported in the hkp "index" response should be the
primary key's creation timestamp. We returned the creation time of the
"primary key signature" (i.e.  newest user id binding signature) before.

see https://tools.ietf.org/html/draft-shaw-openpgp-hkp-00#section-5.2
1 job for fix-hkp-creation in 14 minutes and 15 seconds (queued for 1 second)
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passed #375118012
docker
build:binary

00:14:15