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.

dmarcdb/eval.go

@@ -252,7 +252,7 @@ var jitterRand = mox.NewPseudoRand()
 // Jitter so we don't cause load at exactly whole hours, other processes may
 // already be doing that.
 var jitteredTimeUntil = func(t time.Time) time.Duration {
-	return time.Until(t.Add(time.Duration(30+jitterRand.Intn(60)) * time.Second))
+	return time.Until(t.Add(time.Duration(30+jitterRand.IntN(60)) * time.Second))
 }
 
 // Start launches a goroutine that wakes up at each whole hour (plus jitter) and