storepool: set last unavailable on gossip dead

Previously, the `LastUnavailable` time was set in most parts of the
storepool when a store was considered either `Unavailable`, `Dead`,
`Decommissioned` or `Draining`. When `LastUnavailable` is within the
last suspect duration (30s default), the node is treated as suspect by
other nodes in the cluster.

`LastUnavailable` was not being set when a store was considered dead due
to the store not gossiping its store descriptor. This commit updates the
`status` storepool function to do just that.

Informs: cockroachdb#98928

Release note: None

pkg/kv/kvserver/allocator/storepool/store_pool.go

@@ -267,6 +267,7 @@ func (sd *StoreDetail) status(
 	// even before the first gossip arrives for a store.
 	deadAsOf := sd.LastUpdatedTime.Add(deadThreshold)
 	if now.After(deadAsOf) {
+		sd.LastUnavailable = now
 		return storeStatusDead
 	}
 	// If there's no descriptor (meaning no gossip ever arrived for this