switch to math/rand/v2 in most places

this allows removing some ugly instantiations of an rng based on the current
time.

Intn is now IntN for our concurrency-safe prng wrapper to match the randv2 api.

v2 exists since go1.22, which we already require.

queue/queue.go

@@ -1370,7 +1370,7 @@ func deliver(log mlog.Log, resolver dns.Resolver, m0 Msg) {
 			return fmt.Errorf("get message to be delivered: %v", err)
 		}
 
-		backoff = time.Duration(7*60+30+jitter.Intn(10)-5) * time.Second
+		backoff = time.Duration(7*60+30+jitter.IntN(10)-5) * time.Second
 		for i := 0; i < m0.Attempts; i++ {
 			backoff *= time.Duration(2)
 		}